The Phylogeny of Malel/Female Differences in Sexual Behavior

Medicus, Gerhard, & Hopf Sigrid
Place PublishedNew York
Extentpp 122 - 149
Type of WorkScience, Biology

This is Chapter 4 of Jay R. Feierman, Editor; Pedophilia, Biosocial Dimensions; Springer Verlag New York, 1990


[Page 142]
This chapter offered a biology-based contribution toward a better understanding of the male/female dimorphism of human sexual behavior.
This dimorphism helps in explaining why adult human sexual behavior with children and adolescents is almost exclusively an adult male phenomenon.
However, the views elucidated in this chapter should by no means be misused to excuse or justify socially insensitive behavior toward any adult female, any child, or any adolescent or to reinforce socially unjust male dominance.
Rather, biological knowledge that clarifies human behavioral predispositions should encourage the prevention of negative behavior and enhance the personal and social aspects of human relationships.


[Page 122]
In order to understand human behavior, including adult human sexual behavior with children and adolescents, one cannot neglect humans' phylogenetic [*1] heritage, part of which is the result of the evolution of behavior through natural selection.

  • [*1 -Phylogenetic" (adjective) or "phylogeny" (noun) means "the change of species by mutation and selection during natural history."

This chapter examines the phylogenetic bases of differences in sexual behavior between the sexes. Especially in the Introduction, it is intended to familiarize readers from other disciplines with some of the fundamental evolutionary-biology principles of male/female differences in sexual behavior.

[Page 123]
Also, it is necessary to consider the phylogenetic reciprocal causes of male and of female sexual behavior in order to better understand adult human sexual behavior with children and adolescents. the topic of this edited volume.

Many social scientists attribute most behavioral differences between males and females to the differing processes and influences that each sex experiences during socialization (for further references. see. e.g. Maccoby and Jacklin. 1974). Although it is true that males and females are raised differently by their parents and are treated
differently by society. certain basic biology-related differences in behavior between the sexes do exist. and these differences can best be understood through a phylogenetic approach.

Behavioral scientists and clinicians fmd it difficult to decide on the "degree" to which any human trait, such as objectively observable behavior

  • or subjective inner lif, is determined either by closed (i.e.. innate) genetic programs
  • or by open genetic programs (i.e.. mainly [*2] learned by the individual).
  • [* 2] The term "mainly" is an important qualifying term, since no learning program can be completely open or closed. Likewise, different species' predispositions for ease of learning the same task show that even simple learning is processed through nerve tissue. The structure of nerve tissue is genetically determined and is under the influence of natural selection.

Here, also, a comparative phylogenetic approach can help.
Certain behaviors, such as the facial expressions associated with laughing and crying, are seen in all known human cultures (e.g.. Eibl-Eibesfeldt, 1967, 1984), and variations of these traits are seen in some nonhuman, higher primates (e.g., van Hooff, 1972; Jolly, 1972).

In cases where a behavioral trait is seen in numerous closely related species, it is highly probable that the behavior is regulated by relatively closed genetic programs in the form of fixed-action patterns [*3] that can be traced back to similar genetic material acquired from a common archetypal ancestor.

  • [* 3] - "Fixed-action patterns" are movements that have an innately constant
    form and need not be learned, such as a spontaneous smile.

Examples of behavior patterns that are controlled by relatively closed genetic programs also are revealed by ethological deprivation experiments and by research on twins and on certain psychiatric disorders within families.

Other behavioral traits may be regulated at an initial level by seemingly closed genetic programs but then can be modified by individual learning (i.e., relatively open genetic programs). These latter learning programs are advantageous because they allow the modification of innate characteristics to occur much more rapidly than occurs through genetic mutation and natural selection.

Under natural and seminatural conditions, the modification of behavior through learning almost always results in the adaptive modification of behavior, [*4] implying that a large number of genetically based learning programs are present. 

  • [* 4] Lorenz (e.g., 1978) defines learning as a modification of behavior that is almost always adaptive.

Simply stated, genetic programs structure what is learned and how easily and where, when, and how learning takes place. These programs can also be called "predispositions," or "tendencies to learn," and must be considered in the context of both normative (species and sex-typical) and variant (species- and sex-atypical) behavior.

For example, anyone who has observed the behavioral development of kittens and puppies can appreciate how different they are as species in spontaneous play and in the ease with which they learn various things. Similar differences, although more subtle, can be found when one compares the development of a male or a female of the same species.

Natural History

[Page 124]
Darwin was the ftrst person to interpret the variation among species as being the result of "natural selection" working on individual "variation" within a species. Modern geneticists have corroborated a part of his theory at the molecular level. A standard sequence of base-pairs of genetic material contains instructions for the morphology and the physiological function of an organism.

According to genetics, random mutations in the sequence of base-pairs may cause new variants (mutations) in the morphology and/or the physiological function of the organism. Subsequently, natural selection works on the different individuals. Mutation and selection create variation within a species and ftnally among species. Mutations also affect the morphology of the nerve tissue itself, as well as the physiological processes within nerve cells and tissues, creating behavioral variation among individuals.

Evolution (both morphological and behavioral) of species thus has two origins:

  • first, random mutations and
  • second, natural selection.

In selection, e.g., by environmental factors, a mutant is furthered or hindered in the population by the relative number of offspring carrying the mutation who survive and reproduce. This principle of mutation and selection holds true for asexual as well as sexual reproduction.

In trying to understand organisms and their behavior, science must heed an important rule in biology: it must take into consideration both the currently perceivable adaptations of organisms and the phylogeny of these adaptations.

For example, [... ... ... ... Here follows a series of animals ...] [* 5]

  • [* 5] '''Fitness'' means "the sum total of capabilities that give an individual a reproductive advantage over other individuals." Adaptations can be considered to be the components of overall fitness.

Preconditions (i.e., older traits) influence phylogenetically more recent behavioral traits of organisms. Once fmnly established, traits (e.g., behavioral traits) usually are retained in some form in subsequent branches of the phylogenetic tree (e.g., Riedl, 1975).
[Page 125]
Relationships among animal species can be recognized and a phylogenetic tree can be established only on the basis of the continuity of morphological and behavioral traits. When similar traits of different animal species are traceable back to a common ancestor (e.g., any vertebral column to a basic vertebrate), one speaks of homology.

Similar genetic material is the basis for homologies, i.e., similar traits. Also, homologies have similar morphology or similar behavioral structure. They can be understood independently of their function, which occasionally changes in phylogeny, even among closely related species.
Conversely, when similar traits in different species have evolved independently in that they have a similar function, one speaks of analogy.
For example, camera eyes in vertebrates and cephalopods (e.g., cuttlefish) or the legs of insects and tetrapods developed convergently, or in other words, independently (i.e., analogously). They are not traceable back to a common ancestor.

The common ancestor of mammals and birds (a now-extinct reptile) did not show the trait of brood provisioning. (This conclusion is based on the study of extant reptiles.) Therefore, brood provisioning developed analogously in mammals and birds. Since billing and kissing between adult mates developed from brood provisioning, billing and kissing in birds and mammals also are analogous developments. Analogies can be more important than homologies in the effort by science to derive specific natural laws concerning adaptation to certain conditions (Lorenz, 1974).

Inference from Animal to Human Behavior

Two pitfalls should be avoided in comparative phylogenetic research:

  • direct inference from animals to humans and
  • anthropomorphism in theinvestigation of the behavior of animals.

Important considerations in comparative phylogenetic research are

  • (a) From which animals and in which respect may one draw conclusions to humans?
  • (b) What changes may have occurred in previous (retained) behavioral traits under selective conditions that were created by new behavioral traits?
  • (c) On the basis of which previous behavioral traits (or phylogenetic preconditions) did which new qualitative behavioral traits of the nervous system emerge?
  • (d) Which traits are traceable back to a common ancestor (Le., homology), and which developed independently (Le., analogy)? (See, e.g., von Cranach, 1976.)

Inference from one particular species to another one (e.g., from one particular nonhuman primate to humans) should be made only with reservations. If similarities between species can be observed, it is important that science try to understand the phylogenetic as well as the functional causes of these similarities (see Dienske, this volume).

In regard to some disciplines, such as human psychology, it is necessary for science to further elucidate the phylogenetic basis of behavior (also called ultimate causes) if science is to understand behavior. Proximate causes are life events that release a behavior or that cause a modification of a behavior within the
[Page 126]
phylogenetically caused framework. The distinction between ultimate and proximate cause can be useful for a better understanding of both normal and variant behavior.

Biology of Reproduction

All species die out if their individuals fail to reproduce, be it sexually or asexually. However, reproduction does more than counter the loss of individuals. Reproduction allows change over generations to occur. It is a prerequisite for phylogenetic development.

Any type of reproduction requires a mechanism for genetic duplication. In the duplication process, mistakes in the genes (i.e., mutations) can occur. As described previously, mutations result in individuals with variations (i.e., mutants). In asexual reproduction, phylogenetic development (i.e. evolution) occurs exclusively by mutation and selection.

Sexual reproduction adds a factor to mutation and selection that furthers phylogenetic development, in that in sexual reproduction, genetic material from two individual parents becomes recombined in the offspring; new variations occur by this recombination alone.

Natural selection then acts on the new variations caused by mutations and recombination. Each somatic cell (body cell as opposed to egg or sperm) of metazoans (multicellular animals) contains genetic material in duplicate. That is, in every somatic cell, all of the different chromosomes are present in identical pairs (i.e., chromosome pairs-the diploid number of chromosomes).

Each metazoan parent's germ cells (eggs or sperm) contain only one-half (one side) of each parent's chromosome pairs, however (i.e., one of each chromosome -- the haploid number of chromosomes). In fertilization, the germ cells of the parents combine, producing in the offspring the total amount of genes, the usual diploid number of chromosomes (i.e., a recombination of genetic material).

Mutation and recombination establish more variety in genetic patterns than mutation does alone; and in recombination, genetic material is tested from one generation to the next in various combinations.

Then, within the genetic variety that is caused by mutation and recombination, more variants will survive in comparison to a similar amount of variants caused by mutation alone. This is because genes have already been tested in the parents, although in another combination. One can therefore say that the repertoire for (trial and) error, or the field of lethal possibilities, is narrowed within the variety caused by (mutation and) recombination.

Sexual reproduction is advantageous in comparison with asexual reproduction for several reasons.

One is that the intrinsic process of sexual reproduction, i.e., the recombination of genetic material, creates variety that survives and reproduces faster than it is created in asexual, nonrecombinant reproduction, in which mutation is the only source of variation (cf. Rechenberg, 1973). More variants, caused by a higher rate of mutation, can
[Page 127]
endanger, rather than enhance, the survival of a species, since most mutations are deleterious rather than beneficial. Therefore, sexual reproduction allows adaptation to a changing world and phylogenetic development to proceed faster (Bell, 1985; Michod and Levin, 1988), with more safeguards to the integrity of species. than occurs by means of asexual reproduction.

Another advantage of sexual reproduction is "heterozygosity."

  • In homozygous cells, which are produced by inbreeding over several generations, both halves of the chromosome pairs are identical, i.e., the genetic material is the same in each chromosome of a given pair.
  • In heterozygous cells, the halves are not identical. Empirical evidence suggests that the heterozygosity of diploid somatic cells works as a genetic security mechanism, diminishing the effect of potentially disadvantageous recessive mutants (e.g., Lewontin, 1974).

During phylogeny, the benefits of sexual reproduction must outweigh the costs. Otherwise, the species reproducing sexually will not survive. An example of costs to metazoans is the necessary development of the ability to recognize a mating partner who is of the other sex and of the same species.

During phylogeny, however, as soon as a species develops brood provisioning, individuals of the species are no longer distributed randomly within a population from birth, and as a result, mother/young affiliative bonds make inbreeding between mother and young, as well as among the offspring, more likely.

The potential disadvantages of mother/young affiliative bonds for sexual reproduction, i.e., the potential creation of homozygous somatic cells in the next generation, are overcome by costly inbreeding-avoidance mechanisms that also develop during phylogeny (see Pusey, this volume). These mechanisms become as important to individual survival as is sexual reproduction itself (Bischof, 1975, 1985; Shepher, 1983).

Mutation and recombination are the primary causes of diversification in the animal kingdom. However, isolation caused by geographical distance, by adaptation to different ecological niches, and by sexual selection (which will be discussed subsequently) also result in substantial diversity, as evidenced by the many different species and animal groups.

In short, the main origins of phylogeny are

  • mutation,
  • recombination,
  • selection, and
  • isolation.

The Evolution of Mating Effort: Sexual Attraction and Courtship -- A Form of Communication

Sexual behavior certainly evolved within the context of reproductive behavior (but is only one facet of reproductive behavior). Reproductive behavior in its biological sense has two components: mating effort and parental investment. This subsection considers the evolutionary mechanisms underlying mating effort. Parental investment is discussed in several other chapters in this volume

Selection is affected not only by the environment, which is external to the species, but also by other individuals within the species; the latter phenomenon is called "intraspecific selection" (e.g., Mayr, 1972). Sexual selection is a special case of intraspecific selection. Intraspecific communication is both a means for this intraspecific selection and a product of it.

Intraspecific communication emerged from a phylogenetic process called "ritualization." For example, in geese, when an initiator moved to take flight, the intention movement took on a communicative function for the recipient group members during phylogeny. In this way, a common takeoff among members of a flock became inducible. Behavioral phenomena in an initiator or a signaler take on communicative meaning for a recipient.

Wickler (1970) calls this occurrence "semantization by the receiver." Further mutations and selection may then lead to improved communicability, e.g., increased clarity in the initiator's signal. Other examples of ritualized behavior that are derived from sexual gestures became gestures of dominance and submission and vice versa (see Eibl-Eibesfeldt, this volume).

Mood and its outward behavioral expressions (i.e., affect) in an initiator must be phylogenetically tuned to the stimulus filter in the recipient so that the initiator's expression and the recipient's impression conform to each other (Leyhausen, 1968). In this phylogenetic process, expressive behaviors can become modified, and their meaning or function can change (e.g., van Hooff, 1972).

One example, which will be developed in detail later in this chapter, is that many human adult females, who are smaller in size than their male counterparts, also are less boastful in their behavior (especially during courtship) than many males. These characteristics of the females are attributes of human children and adolescents, as well.

Correspondingly, many courting human males expect such attributes in potential love and sexual partners. These expectations are one reason for the relative ease with which pedo- and ephebophilia can develop in adult males.

  • (It must be noted that the degree of sexual differences in appearance and behavior just mentioned can vary substantially between cultures and even within cultures, depending on status, environment [urban or rural], and other factors.)

Mutations and intraspecific selection may on occasion lead to the evolution of traits that become disadvantageous, sometimes even so extremely disadvantageous that the survival of the species as a whole is endangered because of external selection (e.g., Lorenz, 1978).

One example of this phenomenon is seen in the Oreat Argus's (Argusianus argus, a pheasant) courtship behavior, a type of intraspecific communication. The female of the Great Argus prefers to court males with exceptionally large wing feathers. Thus, during phylogeny, these wing feathers have developed to such an extent that the males of the species are almost unable to fly (e.g., Lorenz, 1978).

An example [... ... ...]

As a general principle, the overall behavioral repertoire of a species is fairly similar in both sexes (Ploog, Hopf, and Winter, 1967). Behavioral dimorphism between the sexes is largely characterized by a differing frequency, intensity, [*6] and sequential use of certain behavior patterns.

  • [* 6 -For example, differences in the intensities of combinable moods. Combinable moods are discussed later in this chapter.

This circumstance is similar within all vertebrate species (e.g., Wickler, 1969). In humans, the frequency distributions of types of behavior that are demonstrated by both males and females overlap greatly, and there is more variance between the sexes in atypical sexual behavior than there is in typical sexual behavior. Among individuals displaying atypical, or variant, sexual behavior, males are overrepresented (this topic will be discussed later in this chapter).

Male/female differences in sexual behavior in human and nonhuman primates result from (phylo-) genetic predispositions and from cultural modifications and amplifications. The bases of human dimorphism in sexual behavior and generalizations about them are highly controversial, and therefore, the biological evidence of these bases and the sociopolitical implications of this evidence must be considered (cf. Hun, 1972; Lee and Steward, 1976).

This chapter deals with sexual behavior in humans. It therefore does not concentrate on the intricate manifestations of the large variety of male/female differences in sexual behavior in the animal kingdom, which has been reviewed by others (e.g., Wickler and Seibt, 1983).

However, the next part of this chapter discusses the phylogeny of some aspects of nonhuman sexual behavior insofar as that phylogeny could have relevance to humans. This discussion leads into the third part, which addresses certain human sexual behavior patterns and their variations, some of which can be better understood through the consideration of phylogeny.

On the Phylogeny of Male-Female Dimorphism

Generally, there are two sexes in both plants and animals, and each sex shows sex-specific traits. It will be demonstrated that the differences [Page 130] between the sexes (see Table 4.1 [*]) started with the inception of sexual reproduction in metazoans

  • (e.g., Bell, 1982; Hoekstra, 1980; Parker, 1978, 1982; Smith, 1984; Stearns, 1987; Trivers, 1972; Wickler and Seibt, 1983).

[Explanation of the Table:] [Page 131]

Idealized diagram of some milestones of the phylogeny of human sexual dimorphism. The broad variety of dimorphic developments in the animal kingdom, caused by different functional/historical conditions, is not considered in this table.

Sexual dimorphism can differ substantially in closely related species.

Therefore, human sexual dimorphism apparently did not develop in the unilinear fashion that is shown in this idealized table. The phylogenetic spiral line symbolizes the reciprocal causes of dimorphistic developments prompted by both sexes.

The shaded stripes symbolize the probability of the maintenance of old phylogenetic

  • (In this respect, this diagram is similar to the diagram of a "phylogenetic tree of behavioral capabilities" in Medicus, 1987, which is based upon Lorenz, 1973.)

But remaining characteristics can become modified by different functional conditions (or by changes of functions), e.g., they can be prompted by phylogenetically newer characteristics superimposed on the older preconditions.

But sometimes, phylogenetically older characteristics may become more apparent in some variants of human sexual behavior. See text and Eibl-Eibesfeldt, this volume, for further explanation.

[Page 130 continued]
In order for two kinds of germ cells, or gametes, to join successfully and flourish, two conditions must be fulfilled:
1. There must be a high probability that the two germ cells will meet, and
2. A certain minimum size must be reached after fertilization to ensure a good chance for further development.

These conditions are best fulfilled when one sex produces a small number of large, nutritious germ cells while the other sex produces large numbers of small, and in some species highly mobile, germ cells.

A species with such heterogamy (different-size germ cells) will outreproduce a species with isogamy (same-size germ cells).

As a result, heterogamy is ubiquitous in metazoans. The large, nutrient-rich germ cells (i.e., egg cells), produced by the female [*7], secure the chance for life to start. The smaller but numerous germ cells (i.e., sperm), produced by the male, increase the probability of direct joining with the large cells of the female because sperm are produced in surplus and are mobile in many species.

  • [*7] The sex producing the lesser number of the larger germ cells is by
    definition the female.

If all germ cells were as large as egg cells (i.e., isogamy), an insufficient number would be produced, and therefore, not enough fertilizations could take place. If, on the other hand, all germ cells were as tiny as sperm (also isogamy), the cells, even if they were fertilized, would be lacking in nutrients and would have less chance for further development.

Despite the production of vast numbers of germ cells by the male (which allows, for example, the external fertilization that is characteristic of most fish), the male's expenditure of time and of energy in reproduction usually is many orders of magnitude smaller than are such expenditures by the female.

More recently, in phylogenetic terms, additional expenditures such as long gestational periods and suckling of the young have been required of the female placenta1ia (i.e., vertebrates with placenta).

Because males of most species make relatively small expenditures in time and energy in the effort at reproduction, it follows that a male can mate more often than a female and sire many more offspring than one female can bear.

This capacity means that a male can improve his reproductive success by mating with more than one female within a certain period of time, whereas a female, once pregnant, cannot bear more offspring by mating with more males.

This basic sex difference in reproductive physiology is the foundation of many of the fundamental differences in male and female sexual behavior. In general, the resources limiting female reproduction are mainly nutrition and time: to produce eggs, to nourish a fetus, to nurse the young. In contrast, almost the only resource limiting male reproduction in many species is the availability of the female.

The Concept of "Functional Proximity" of Mood and of Behavior

Because the female represents the limiting resource for the male, males are biologically predisposed to search for and court female partners more actively and intensely than females search for and court males. This difference in sexual behavior leads to intense male competition for females

  • (e.g., Symons, 1979; Trivers, 1972; Wiclder and Seibt, 1983).

In species in which males expend more energy in parental care than do females (e.g., sea horses or such birds as ostriches, hornbills, and lyrebirds), the females court the males more intensely than the males court the females (e.g., Wiclder
and Seibt, 1983).

As Eibl-Eibesfeldt (this volume) [*] shows, the consequence of males' searching and competing for females is a sexuality based on male dominance and female submission (which only later in evolution was overlaid by an affIliative sexuality).

In species where males compete for females, males have an advantage if they are able to switch rapidly from courtship behavior to male intrasexual competitive aggression and vice versa. This ease of behavioral change is reflected in the nominative expression "functionally proximate behavior." As defmed in ethology, patterns of functionally proximate behavior occur frequently within relatively
short periods of time

  • (cf. Baerends, 1976; Lorenz, 1978; Wiepkema, 1961).

Behavior can be combined in very different ways (i.e., can occur simultaneously or alternatingly) and to very different degrees, depending upon the species, the sex, and the context. Because of the human capacity for self-assessment by introspection and reflection, humans realize that this principle also applies to human "inner life," e.g., the moods (self-perceived as feelings) underlying human behavior. Because of such combining, for example, some functionally proximate moods, such as sex and hostility, can become superimposed somewhat easily in males of many species.

The mood for flight from a predator is relatively distant from the mood for courtship behavior on the continuum between functionally close and functionally distant behavior-inducing moods; therefore, a change from flight to courtship (e.g., Lorenz, 1963), the respective behavior patterns that result from these moods, takes much longer to occur than does the change between functionally more proximate moods or behavior patterns (Lorenz, 1978). Life-threatening shortness of breath is another example of an extreme state that precludes any quick switch to a predisposition such as sex or hunger.

The closely related, sometimes simultaneous aggression and courtship behavior patterns of male intrasexual competition have evolved into differingkinds of behavior in the vertebrate classes. Beatrice Oehlert (1958) was the first to write about this relationship, citing cichlid fish (Cichlasoma biocellatum and Geophagus brasiliensis) as an example: [Page 133]

  • "Courtship behavior often begins with hostility. When a male is defeated, it immediately loses its interest in courtship behavior and flees. In comparison to this, a submissive female is, despite all readiness to flee, able to perform sexual actions; but the female never shows courtship behavior when she is dominant. If two males meet, they do not pair, because the weaker flees immediately. Two females do not show pairing behavior towards one another . . . [i.e., the dominant one cannot be in a sexual mood at the same time as she is dominant]" (p. 169).

Thus, homosexual pairing is rare, because to engage in courtship behavior, a male, in certain species, must be dominant, and a female must be submissive.

Male/female differences in behavior are especially relevant in species in which the sexes are identical in outward appearance. Certainly, male/female dimorphism in sexual behavior does not show the symmetrical complementariness between the sexes in all vertebrates as is shown in the cichlid fish cited by Oehlert (Table 4.2) and only sometimes functions as a mechanism that prevents homosexual pairings.

Explanation of Table 4.2:  [Page 134]

In most buman males and females, sexual arousal usually is inhibited by real fear or strong hostile aggression. However, in a relatively few human individuals, sexual arousal can be facilitated by these emotions. Within this population, hostile aggression facilitates sexual arousal almost exclusively in males.
In addition, anecdotal evidence indicates that fear can facilitate sexual arousal in more females than in males, although most likely only in a few (sometimes pubescent individuals) (cf. Eibl-Eibesfeldt, this volume [*]).

It appears that in humans, the facilitation of sexual arousal by fear or hostile aggression is dependent upon
(a) sex,
(b) age,
(c) situation and context of fear or aggression, and
(d) individual differences, e.g., responses learned by the individual within
the biological framework. (See text for further explanation.)

A phylogenetic heritage might help to explain why, within this population, hostile aggression facilitates sexual arousal more commonly in males and fear most likely has a similar effect more commonly in females.

[Back to Page 133]
In many other species, display behavior (an aspect of courtship) is shown almost entirely by the male, from the very beginning of courtship behavior onwards, and this behavior serves both to intimidate other males and to attract females.

According to Lorenz (1963), Oehlert's observations can even provide a basis for the understanding of some aspects of gender-based dimorphism in human sexual behavior

  • (see also Bischof, 1979; Eibl-Eibesfeldt, 1967; Medicus, 1987).

Lorenz (1963, Chapter VI) [*8] wrote:

  • "Two motives, which in one sex scarcely inhibit each other, exclude each other in the other sex in a sharp shunting mechanism."
  • [* 8] lIn the English version, On Aggression, Reprint 1972, p. 89.

This topic also is discussed in Human Ethology by Eibl-Eibesfeldt (1984). (See also Eibl-Eibesfeldt, this volume. [*])

The observations by Oehlert gave the fIrst indication that male/female dimorphism of emotions might better be understood by the considering of phylogeny. A search in this direction in turn may shed light on some variants in human sexual behavior.

In a stable hierarchical relationship, aggression is linked to dominance more than to submission, and fear is linked to submission [*9] more than to dominance (see also Eibl-Eibesfeldt, this volume), although many dominance/submission relationships show (almost) no aggression or fear.

  • [* 9] Aggression is a behavioral tool used, for example, in gainig dominance. In many species (for example, chimpanzees; for further references, see, e.g., Goodall, 1986), changes in the dominance/submission relationship can occur in relation to estrus; and in many species (e.g., many birds) in which males and females provide brood provisioning, the dominance/submission relationship changes during the period of provisioning.

Also, in many species, even when male courtship and male aggressive competition are not strongly linked with each other, they nevertheless may not inhibit each other.

Body-Size Dimorphism

Understanding the complex selection mechanisms that regulate size differences between the sexes of a species is a difficult task (Frayer and Wolpoff, 1985). Larger females within a species raise relatively more infants than do the smaller females (cf. Lewin, 1988; Ralls, 1976).

[Page 134]
This tendency (i.e., larger females = more offspring) evidently could not counteract the trend of selection for larger males than females in many mammals, however. In polygynous species, though, as a rule, [*10] the male is larger.

  • [*10] Here, one exception is the Weddell seal.

This dimorphism exists because, in the face of marked male competition for females, strong, large, and dominance-seeking males usually develop during phylogeny.

One example of unimale-polygyny-induced male largeness is the sea elephant, a species in which the males are up to four times as heavy as the females.

Other examples of extreme male/female dimorphism in size are unimale-polygynous hamadryas baboons, orangutans, and gorillas, in which males are approximately twice as heavy as females

  • (Harcourt et al., 1981; Hrdy, 1981; Kummer, 1968; Martin and May, 1981; Mitchell, 1979; Poirier, 1972).

Male body size in these mammals appears to be related to the degree of male competition for the female.

Male body size also can be influenced phylogenetically through female
sexual selection

  • (Bateson, 1983; O'Donald, 1967; Trivers, 1972, 1985), [Page 135]

an intraspecific kind of selection that results from the females' choice of mates. The male descendents of a female who has chosen a large, strong, and dominant male as a mate will have a selective advantage, because large and strong males are more likely to beget large and strong sons capable of intimidating other males.

As a result of this intraspecific selection during phylogeny, in many species the male impresses the female by sumptuous and seemingly overenergetic shows of dominance and power (a characteristic that is present, for example, in many mammals and birds).

In contrast to the trend of strong males, females of many species (especially polygynous species) have taken on opposite or complementary traits. As mentioned previously, they are smaller and weaker than males and sometimes bear more resemblance to a nonadult than they do to an adult male.

As a result of phylogeny, this kind of resemblance also exists in human females, and also based on phylogeny, small size in human females is an attribute that many human males find attractive. It is possible that because of these characteristics of human females, certain traits exhibited by children and adolescents sexually stimulate some adult males. This realization may be important in helping science to understand adult human sexual behavior with children and adolescents.

Human Male-Female Sexual Dimorphism

As was just noted, the human species is mildly dimorphic sexually. The average human female is smaller in body size than the avemge human male and has a weaker musculoskeletal build than the male. This dimorphism may have had its origin in polygynous mating systems. Its continuation, however, could be the result of several other factors, one candidate being the division of labor.

For example, a strong male, especially one that is of high rank, potentially can provide better protection and resources than a weaker male can. The importance of female sexual selection for these traits is evident. Many adult males try to impress adult females by boasting about their strength and status, whereas the courtship behavior of adult females often invites friendly interaction and is sometimes associated with submission.

Also, in human reproduction as in the reproduction of many other species of animals, most fathers expend less time and energy than do most mothers. This chamcteristic is one result of selection (but can be modified by culture). However, the characteristic may not be of great significance in terms of the phylogeny of humans, because male/female differences in expenditures of time and energy are relatively more important in species in which fathers do nothing more than sire the children.

Paternal contributions to the care of offspring have increased in humans during more recent phylogeny, a trend that has not occurred in other extant hominoids (apes). Such contributions can vary widely from
[Page 136]
virtually no investment to almost complete care of the offspring.

Paternal investment is manifested by social activities such as sharing food, caring for and protecting the family, and teaching and playing socially with offspring

Offspring receiving biparental care generally are more advantaged than are offspring who lack such care. This comparatively large male contribution to parental investment (relative to other hominoids) may be one reason why certain sexually dimorphic behavioral traits are phylogenetically less distinct in humans than they are in some other primate species.

An additional reason may be the previously mentioned compatibility of moods. Male protection of a female and offspring is compatible with externally directed threat, but threat is not appropriate between the protector and the protected. Protection should not be frightening to the protected (cf. text to Table 4.2). [See here above]

The phylogeny-based capacity to establish long-lasting monogamy appears to be related primarily to biparental care. This capacity is especially relevant under ecological conditions in which brood provisioning cannot be accomplished as well, or even at all, by a single parent.

The reproduction related behavior of many species of birds exemplifies this relationship. In many species, both parents are needed for the successful provisioning of the offspring and are active in the effort, and the parents pairbond. Male mammals lack functional mammary glands and, consequently, cannot contribute to feeding the sucklings during their most dependent period of life. Accordingly, mammals pair bond relatively rarely compared to birds.

One example of monogamous mammals is marmosets, New World monkeys the males of which contribute greatly to the raising of the young by carrying the infants

  • (e.g., Welker and Schafer-Witt, 1987).

From the perspective of sociobiology or socioethology, parental investment types of behavior make sense only if adult males and adult females raise offspring to whom they are closely related, because the effort will enhance the survival of the genes of the offspring-and consequently, the genes of the adults. Usually, there is no question that females (e.g., female mammals) raise their young.

This principle is not always true for males, however, but it can become increasingly so as the paternal certainty of males increases. A male can increase his paternal certainty by monopolizing an individual female for at least one period of estrus, as occurs, for example, in sea elephants and sometimes in chimpanzees.

In mammals, both brood provisioning by the female and the sexual monopolization of the female by the male were phylogenetic preconditions for the development of contributions by males to brood provisioning. This behavior then resulted in more permanent male/female bonding and, also, allowed a female to monopolize the paternal investment for her own family.

In human societies, adult females and males often direct some of their parental investment beyond their own offspring, contributing toward the rearing of their sisters' and brothers' offspring, with whom they share, on average, 25% of their variant genes.

  • (Concerning avuncular relationships, cf. Chagnon and Irons, 1979; Eibl-Eibesfeldt, 1984.)

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As stated previously, adult/adult pairbonding (i.e., affIliation, attachment) is relatively rare in mammals as compared with birds. Yet, as has been shown, adult male/female pairbonding does develop in mammals to varying degrees. This statement applies to humans, in which the degree of such bonding is shown by a variety of expressions from monogamy through polygamy. Attempts to categorize the degree of pairbonding in all humans requires an oversimplification, because there is considerable inter-societal variance in both monogamous and polygamous relationships.

For example, even though monogamy is professed through socially sanctioned marriage, it is not necessarily strictly followed. Conversely, in cultures in which polygyny is permitted, it is realized in a relatively small percentage of adult males, who command adequate resources to procure and support several wives.

  • (Polygyny [one male/many females] is found more often than polyandry [one female/many males]).

Also, male/female bonds of affection do not necessarily last a lifetime, and as one bond ends and the next begins, a pattern of serial monogamy is produced in some societies.

Some aspects of human mating systems can be understood by means of cross-cultural comparisons

  • (e.g., Ford and Beach, 1951; Symons, 1979).

Lifelong and relatively strict monogamy does exist in some cultures

  • (although perhaps somewhat at odds with the nature mainly of human males),

and it can be interpreted as being a cultural adaptation that increases cooperation in the rearing of young, [*11] facilitates the avoidance of social conflicts among families, or as proposed in more recent literature, helps to prevent the contraction of sexually transmitted diseases.

  • [* 11] Advantages of biparental care for social learning of children can remain as a predominant factor, for example, in some social classes of the industrialized world in which mothers are not predominantly dependent upon paternal economic help.

The significance of etho-psychology to the understanding of adult human sexual behavior with children and adolescents is that etho-psychology shows why even married adult males with relatively regular opportunities for sexual intercourse can be vulnerable to sexual attraction, arousal, and behavior with children and adolescents, while adult females are less vulnerable.

Etho-psychology demonstrates why many adult males who actually are married and have offspring of their own

  • (Alter-Reid et al., 1986; Daly and Wilson, 1985)

can become sexually involved with children and adolescents. Also, ethopsychology can address an interesting question whether maternal feelings affect sexuality differently than do paternal feelings. Again, it must be emphasized that demonstrating why males are predisposed more than females to such behavior in no way justifies relationships that can be detrimental to any partner. The elucidation of the reasons simply helps to explain the phenomenon.

Biopsychology of Human Courtship

Male/female dimorphism of sexual behavior is observable in humans. In general, human males search for a partner and court more "ardently" and
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obviously than do the "reluctant" human females (Symons, 1979).

Also, in common with the probabilities that are pertinent to most other mammalian males and females, human males are more likely to initiate obvious and overt sexual actions than are human females

  • (e.g., Ellis, 1986; Goodman, 1976; Goy and McEwen, 1980; Masica, Money, and Ehrhardt, 1971; Michael, 1968; Money and Ehrhardt, 1972; Money and Mathews, 1982; Money, Schwartz, and Lewis, 1984; Symons, 1979).

In addition, masturbation is more likely to be self-discovered by young human males after puberty than by young human females. Once the behavior is discovered, young males engage in it more frequently than young females, and more young males practice it than young females

  • (e.g., Kinsey, Pomeroy, and Martin, 1948; Kinsey et al., 1953).

Among humans, males are more [* l2] easily sexually aroused by visual stimuli of anonymous sexually provocative adult females than are females by similar visual stimuli of anonymous sexually provocative males

  • [* 12] The word "more" is used to emphasize that human females, too, are aroused by visual erotica (although usually not as much by genital closeups as human males), even though they do not seek it out on their own to the same degree as males
    (Masica, Money, and Ehrhardt, 1971; Symons, 1979; and others).

Also, males are aroused by novelty (e.g., by variety of sexual partners) more than females are. This dimorphism is in accordance with natural selection, because males with such a proclivity have more offspring than males without it. Apart from the fact that human males often are attracted to specific physical or cosmetic attributes in their female counterparts, many males do not appear to be very particular in their choice of sexual partners.

This broad-mindedness exists because males do not necessarily expend much time and energy in reproduction. Clearly, a female who mates with many males will not increase her fitness as much as a male who mates with many females

  • (e.g., Freedman, 1979; Smith, 1984),

because once a female is pregnant, further matings during her pregnancy will not lead to more offspring. In many tribal societies, as well as in much of the "Third World," human females spend most of their fertile lifetime either pregnant or nursing. The male/female dimorphism in the number of offspring that humans can produce makes it likely that male/female differences in biopsychological skills were selected for.

Differences in the desire for polygamy can also be observed by comparing male and female homosexuals. Most sociosexual affiliative bonds between male homosexuals are not as enduring as are those between most female homosexuals. During their lifetimes, female homosexuals usually have far fewer sexual partners than male homosexuals

  • (Hooker, 1967; Kinsey et al., 1953; Symons, 1979).

In the case of heterosexual relations, Symons (1979) emphasized that many human males are likely to establish social bonds (if any) to attain sexual access and that human females are more likely to accept sexual behavior to attain social affiliative bonds. If such an afftliative bond is really established, human males are more likely
to contribute in child rearing. [* 13]

  • [* 13] l'During the phylogenetic development of such a social system with a bond between the parents, a perceivable estrus could have lost importance (cf. Benshoof and Thornhill, 1979).
    In addition, a concealed ovulation (i.e., a not perceivable estrus) might have improved the possibility for the female to better control the selection of a mate to bond with. This is especially important for the female because a bigger variance of parental contributions is possible in males than in females. Instead of a temporary estrus (which cannot be controlled by the female and therefore can occur at the wrong time), permanent traits of attractiveness by certain distributions of fat (i.e., cheeks, hips, breasts) have developed.

As is true of other vertebrate males during courtship, many courting human males try to impress females by direct and indirect displays of power, status, and resources. The impressive but usually affiliative behavior
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of human males during the search-and-pursue stage of courtship is a form of ritualized threat that developed phylogenetically from male intrasexual competition. Because of the potential protective ability of males, many females tend to find male strength attractive; they often respond positively to high-status characteristics (e.g., Miller, 1981).

Some body traits of adult human females, such as smooth skin, relative hairlessness, and softness, make them resemble children. Adult males usually have fewer of these kind of body traits or have some of them to a lesser degree. In the human female, traits of smallness, tenderness, and childlike features

  • (cf. "child schema" of Lorenz, 1943; see also Schleidt, 1962)

can be exaggerated by aspects of culture, e.g., by fashion, cosmetics, and learned behavior, to the point that females actually appear to be helpless and in need of male protection.

It is important to emphasize that by no means all males seek small helpless females and that mate selection in many cultures does not emphasize such qualities, although male attraction to childlike features and behavior in females may be an innate tendency.

Another important and contrasting characteristic of attraction in males can
be material qualities and attributes of economic competence.

Human female courtship behavior, such as proceptive flirting behavior, is less boastfully threatening than is the behavioral counterpart in some males and sometimes is associated with submission and insecurity

  • (Grammer, paper in preparation)

rather than with strong competition or aggression. Combinations of nonthreatening behavior with finely tuned interactive behavior often are characteristic of female courtship behavior

  • (Beckmann, 1977; Buss, 1985; Lockard and Adams, 1980; Moore, 1985; Perper and Weis, 1987; Wilson and Lang, 1981).

Despite the previously mentioned male/female behavioral differences in the search-and-pursue aspect of courtship, human females often initiate courtship with a desirable male, e.g., by inconspicuous and brief eye contact, by preening, or by other attention-getting proceptive behaviors

  • (e.g., Beach, 1976; Moore, 1985).

There is much cultural variation in female strategies of competition. Nevertheless, in general, even when they are in competition for a highstatus male, females may use these inconspicuous (and seemingly passive) methods of seeking contact.

Courtship behavior in adult females actually is similar in some ways to the behavior of children and adolescents

and it appears that many pedophiles and ephebophiles misinterpret friendly behavior by children and adolescents as being sexually seductive. Human females frequently control the development of interaction by acknowledging or ignoring male behavior. To exert this control, females use a variety of nonverbal signals

  • (e.g., Givens, 1978; Perper and Weis, 1987).

Such subtle and finely tuned interactive behavior is certainly more developed in most adults than it is in children and adolescents.

[Page 140]
Coyness, [* 14] one adult female response to a male's courtship efforts, may lengthen the period of time in which a mutual decision is made. Human females do not value superficial physical qualities and sexual attractiveness in a potential mate as much as males do.

  • [* 14] Expressions of coyness, which prolong courtship and are exhibited mainly by the female, have a phylogenetic basis: it is a phylogenetically ritualized form of the female conflict between attraction tendencies and flight tendencies (e.g., Eibl-Eibesfeldt, 1967, 1984).
    Because of the different expenditures for reproduction in terms of time, energy, materials, and risk (cf. reproductive strategies) between the sexes, coyness in courtship is now understandable in mammals as being adaptive for females.

In addition to sexual relations, females are more likely to seek affection, as well as the capacity for intimacy, security, and trust and the ability of potential fathers to provide for their offspring (Strassmann, 1981).

Because females seek these returns, the length and quality of acquaintance from which a mutual decision can arise is particularly important to the female during the process of pairbonding. But the adaptational causes of a behavior in general and sexual choice in particular are certainly not always conscious in humans (e.g.,
during courtship).

Biosocial Consequences

The previously mentioned differences between the sexes in appearance and behavior, caused by mutation and intraspecific selection, have consequences that go beyond courtship and sexual behavior. Because of their nonaggressive, nonthreatening appearance and behavior, human females are usually treated as if they were of lower rank, even if they are of equal rank or status with males

  • (e.g., Rosenblatt and Cunningham, 1976).

For example, it is noticeable that both sexes (except a courting male) normally accord all females less attention and status than they accord all males

  • (e.g., Greif, 1980; Thome and Henley, 1975; Zimmermann and West, 1975).

The male's conflict in courtship consists of his keeping rivals at a distance while he approaches the female. The conflict for the female is that the courting male can be perceived either as being threatening or as offering security. In the eyes of a male in a competitive context, the nonthreatening behavior of adult females and young may appear to be a signal of weakness and, therefore, may be perceived as submission, even if the behavior is not intended as such by an adult female partner.

Therefore, there may be a certain phylogenetic basis for the relative lack of status of females and young that is conferred by adult males and females. However, this kind of "submission" can also be seen as a social skill that may be used to de-escalate tendencies toward conflict, to achieve appeasement, and to promote positive social relations.


So far, this chapter has tried to develop a phylogenetic understanding
of why there is a functional relationship between tendencies towards
aggression, fear, and sexual behavior between the two sexes and how the
relationship operates

  • (e.g., Brown, 1981; Cordoba and Chapel, 1983; Donovan, 1985; Eibl-Eibesfeldt, 1984; Gray, 1971; Lorenz, 1963; Marshall and Christie, 1981; Oehlert, 1958; Revitch, 1980; Rubin, Reinisch, and Haskett, 1981; Spengler, 1979; Spodak, Falck, and Rappeport, 1978; Zillmann, 1984).

[Page 141]
As has been pointed out, adult human sexual behavior with children and adolescents can be understood within the context of dimorphism. Male/female differences in sexual behavior result at least in part from the different amounts of energy and time each sex expends for reproduction

This initial difference produces a difference in the functional proximity of the moods of fear and aggression to sexual mood

as well as a difference in the likelihood for active pursuit of sexual opportunities

  • (Ghesquire, Martin, and Newcombe, 1985).

A biopsychologica1 understanding, derived from the study of the pertinent phylogeny, might be useful in the effort to understand why the seduction of an adult sexual partner may be undertaken, in different forms, by both adult males and adult females but why rape and other forms of sexual violence appear to be committed almost exclusively by a very few males.

Ethology helps to provide an understanding of male/female differences in behavior that relate to the frequency of occurrence of variant sexual behavior. By no means does ethology encourage or justify any behavior that harms or takes advantage of a partner (cf. Bischof, 1985).

Like sexual violence, adult human sexual behavior with children and adolescents is almost completely a male phenomenon

  • (e.g., Alter-Reid et al., 1986; Finkelhor, 1979; Lechmann, 1987).

One of the most consistent psychological correlates that has been identified is that many adult males who are sexually attracted to children and adolescents show manifestations of (real or self-perceived) social inadequacy with peers. Such males may suffer from feelings of anxiety when they consider sexual behavior with an adult partner

  • (Johnston and Johnston, 1986; Panton, 1979; Segal and Marshall, 1985).

Because sex and fear are such functionally distant moods in males, many human adult males cannot perform sexual actions when they feel inferior to their sexual partner or when they feel anxious

  • (e.g., Bancroft, 1985; Eicher, 1980).

This explanation is in accordance with the previously mentioned male/female dimorphism in the relationship of mood and behavior (see Table 4.11).

Because of their small size, lack of experience, and sense of insecurity, children and adolescents of either sex do not arouse feelings of inferiority, fear, and anxiety in adult males. Thus, children and adolescents can become "sexual objects" for males who in sociosexual relations with adults feel inferior or anxious.

Furthermore, the similarity of appearance between children and adolescents and adult females (cf. "child schema") makes it easier for the male pedophile or ephebophile to substitute a child or an adolescent for an adult female partner.

Moreover, as mentioned previously, it is adult males who are more likely to search out and initiate sexual behavior than adult females.

[Page 142]
Leaving out the possibility of physical violence, which is very rare, one harmful effect to a child or an adolescent who has experienced sexual activities with an adult might be the absence of the experience of a gradual psychosexual maturation -- that is, the absence of the experience of the many stages of falling in love

  • (Kretz, Reichel, and ZiX:hling, 1987).

The degree to which this deficiency may burden a child's or an adolescent's ability to establish meaningful adult relationships is yet to be determined.

The understanding of biopsychological differences between the sexes (in children and adolescents and in adults) provides a biosocial basis of the appreciation of one's own, as well as others', predispositions.

However, the realization that comes from this understanding should lead to caution in the attempt to project the understanding of one's own sexuality onto the sexuality of children and adolescents who are developing or onto the sexuality of a partner of the opposite sex. Care also should be taken to avoid extrapolating the male/female differences found in courtship behavior and sexual behavior to other types of interpersonal behavior. Other social relationships should be viewed in their own right.


This chapter offered a biology-based contribution toward a better understanding of the male/female dimorphism of human sexual behavior.
This dimorphism helps in explaining why adult human sexual behavior with children and adolescents is almost exclusively an adult male phenomenon.
However, the views elucidated in this chapter should by no means be misused to excuse or justify socially insensitive behavior toward any adult female, any child, or any adolescent or to reinforce socially unjust male dominance.
Rather, biological knowledge that clarifies human behavioral predispositions should encourage the prevention of negative behavior and enhance the personal and social aspects of human relationships.


The References are here: < feirenman_4_references.pdf