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Homosexual males, their brains and evolution

The Puzzle. Exploring the Evolutionary Puzzle of male homosexuality.
by Louis A. Berman
Godot Press 2003 paperback 583 pages.
illustrated, 2 indexes, references.

reviewed by Gert Korthof, 23 Dec 2003   (updated 8 June 2004)

    According to psychologist Louis Berman, homosexual males have low-masculinised brain ('female-like') brains because they have experienced prenatally a lower brain masculinisation (this is the proximate cause). As a side effect of the mechanism that creates the male brain, a small percentage of the male population has a low-masculinised brain, but are otherwise physically normal males. Many of them become homosexuals. Berman places this in an evolutionary context. Homosexuality in human males is a by-product of an evolutionary process that reduced gender differences and changed a harem based social structure to a heterosexual pair system (this the ultimate cause). Therefore, homosexuality has no evolutionary function. Furthermore, the evolutionary transition from ape to human included a 'premature born' baby, which required a caring mother. The mother needed a father who not only delivered food, but cared for the children too. This required the construction of a less aggressive male brain. Variability in the mechanism that produces this new male brain causes a small percentage of males with more 'female-like' brains.

gender differences
slightly modified from Berman (2003)

aggressive maleevolves gentle male
haremevolves male-female pairs
pronounced sexual dimorphismevolves less pronounced
normal babyevolves 'premature' baby

    An attractive feature of Berman's evolutionary explanation is that it connects previously unconnected facts from psychology, brain research, endocrinology, embryology, anthropology and evolution to explain homosexual behaviour. Berman is not a biologist, but if his theory has anything to say about reality, then it should be able to generate predictions beyond humans. Humans are confronted with the same problems all animals are confronted with: surviving, getting food, getting shelter, getting a mate, getting children, feeding children, protecting children, educating children, etc. Humans share up to 98% of their genomes with chimpanzees. Brains have a common ancestor too. Brains did not fall out of the blue sky. That is what evolution is all about. Remember Darwin is on the cover. Therefore, it is inevitable to ask questions such as: Are homosexuality and small sexual dimorphism of body size in animals correlated? Is the occurrence of homosexuality in species correlated with low aggression of the males? Have low-aggression males more offspring? Is it correlated with monogamy? For the human species, one should collect data about the actual amount of time fathers invest in their children and not just assume that this amount has increased. Are human fathers, better fathers than elephant fathers? In humans, do males still prefer smaller females? Do females prefer larger males? Male and female chimps have equal body sizes (7), but chimpanzee males do not generally participate in raising their own offspring. Chimpanzee, orang-utan and gorilla mothers provide all the care for their offspring, but according to Bagemihl the males of the three species show homosexual behaviour (1). Owl monkey fathers carry their infants from almost immediately after birth and provide most care (2). Does male homosexuality occur in the Owl monkey with higher frequency than in species where the mother cares for the children?
From this 'quick and dirty' search in the literature, it seems that there is no simple relation between size dimorphism, male aggression, paternal care and homosexuality in our closest relatives. It could be that the human species is unique, but there are always similar conditions of life in other animals. More data are needed. However, we know at least what questions we should ask.

    It is amazing that Berman missed Bagemihl (1), but on the other hand, Berman produced a theory to explain human homosexuality, so maybe Bagemihl is not extremely relevant for that topic. Bagemihl is good at collecting facts, but his own alternative non-Darwinian theory is not very helpful. Berman's theory is very much richer, but he did not apply it to the animal world. However, from a book that explores the evolutionary puzzle of homosexuality and features Darwin on the cover, I had expected more space for animals. But even when one accepts that the scope is restricted to humans, crucial information is missing: the lifetime 'reproductive output' of homosexuals compared to heterosexuals. Berman has a chapter about homosexual men who marry, but I did not find data about children. Only those data can define the degree of the Darwinian 'puzzle'. If homosexuals on average were to have as many children as heterosexuals, there would be no Darwinian puzzle. It is always important to define your problem exactly before developing a solution.

Reproductionpositive factors negative factors
heterosexuals high fertility, parental care infertility (intersex, sex-chromosome disorders), spontaneuous abortion, perinatal death mother/baby, celibacy, infanticide, sexually transmitted diseases, incest, long pregnancy and lactation period, menopause
homosexuals bisexual with offspring exclusive homosexuality

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The by-product hypothesis

  Berman's by-product theory is different from all other theories that try to explain homosexuality. Not because it is limited to males, but because it claims that homosexual behaviour has no evolutionary function. Homosexuality is a by-product of the variable effect of testosterone on the brain during prenatal development. Berman suggests that natural selection produced a lowering of the average degree of masculinisation of the male brain. Due to naturally occurring variability, the lower end of the normal distribution produces homosexual males.
    This is a pretty sophisticated theory. However, explaining a feature as a by-product is itself not a sufficient explanation. There must be an element of inevitability in the by-product to be a plausible explanation. In general, when a characteristic is useless or harmful for reproduction, evolution theory predicts it will eventually disappear. What prevents natural selection from optimising the mechanism that produces the male brain? Variability can be small or large and have a symmetric or asymmetric distribution. Why is the variability so large that it results in up to 4% homosexual males? Why is natural selection not able to eliminate by-products? The reason generally is that a compromise is the best what can be achieved (sickle cell is an example) and the benefits of the solution outweigh the costs. To evaluate the by-product hypothesis we need to know what the costs and benefits are of an evolutionary lowering of the average masculinization of the male brain. It is not impossible to measure the evolutionary benefits. The benefits must be substantial. For example, do (heterosexual) men with lower masculinized brains on average have more children than men with normal or high masculinized brains? and vica versa? Berman must be interested in this question. If no difference can be found, Berman's hypothesis seems doubtful. Intuitively, the 'evolutionary costs' seem clear, but are harder to pin down. The closest figure what I could find in Berman's book was that nearly 46% of gay men have heterosexual contacts, which suggests that homosexual men have at least a 46% reproductive loss compared to heterosexual men. It seems hard, but not impossible to get better data than that (via indirect approaches). Nevertheless, Berman is very pessimistic: "Unfortunately, one could never collect the relevant data, since homosexuals who marry and have children are, in most cases, closeted and cannot be identified" (5).
    Contrary to Berman's opinion, data about reproduction of homosexuals do exist. A 1994 survey reported that 67 percent of lesbian women were mothers, compared with 72 percent of straight women [only a 5% difference]. In Japan 83 percent of homosexual and bisexual men had offspring. Joan Roughgarden (2004) concludes: "All in all, the data do not support uncritical acceptance of homosexuality as deleterious" (6). Therefore, it could be that the evolutionary puzzle of homosexuality is not as big, as Berman thinks it is.
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Are humans unique?

  Berman uses the following quote of Frank Beach (1978) twice in his book: "Human sexuality is about as closely related to mating behavior of other species as human language is related to animal communication, and that relationship is distant indeed" and he repeated it again in a personal communication to me.
It seems that Berman is committed to the view that humans are unique (3), because his theory depends on it (or the other way around). The prematurely born baby is a unique human feature he says. However, the young of kangaroos and other metatherians are extremely immature at birth. In mammals, bears give birth to very immature babies. Therefore, humans are not unique in this respect. Following his logic, extra maternal care is needed as a consequence of the premature birth. This demands extra paternal support. This demands a gentling of the male brain. This demands a reduction of sexual dimorphism and a lower masculinisation of the male brain. As a by-product a low percentage of homosexual males are born each generation. Yes, all this makes sense, but this view also creates an (unconscious) bias in Berman's thinking. If one or more events in this chain of events turned out to be not unique for humans, Berman must check out what happened in other animals under similar conditions. This could falsify his hypothesis (4). However, we need as many similar cases in animals as we can find. One species is not enough.
    Intriguingly, Berman reports animal data that are, I assume, relevant and support his hypothesis. For example he reports that monogamous male sparrows tend to have lower testosterone levels than polygamous sparrow species. Experimentally increasing testosterone levels stops males helping to feed their young and they start chasing available females. The experimenters reported a precipitious drop in the survival rate of the baby sparrows. This extremely interesting experiment shows that testosterone levels could control monogamy versus polygamy. Furthermore, it shows the effect on the reproductive output. And that is what evolution is all about. To test Berman's hypothesis, it would be extremely interesting to see what the effect of prenatal testosterone treatment is on sexual behaviour later in life, especially in mammals (rats, monkeys, apes). This is the way to go (2).
    Finally, if behaviour and hormones in sparrows were completely irrelevant for humans, then why does Berman report about sparrows at all? If Berman has an elaborated hypothesis about male homosexuality, then it could be justified to ignore animals. However, if he wants to explore the evolutionary puzzle of homosexuality, then either go for it, or remove Darwin from the cover.

    In the meantime, Berman's story goes well beyond explaining male homosexuality. It is a story about how events in our evolutionary past shaped our brains and behaviour. For me, the book triggered a similar question as 'The Aquatic Ape Hypothesis', namely: What environmental (and social) conditions was the human body designed for?

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  1. Bruce Bagemihl (1999) Biological Exuberance, p.276-288. A good hypothesis makes all the difference. For example Bagemihl reports weights of male apes, but not of the females, so we cannot deduce size dimorphism from his data. Further, it is a pity that he did not report about homosexuality in the unusual and interesting owl monkey.
  2. Good examples of comparative brain and behaviour research can be found in John Allman (2000) Evolving Brains, Scientific American Library, p.182. Reviewed in Science, Vol 283, issue 5405, 1121, 19 Feb 1999.
  3. In an email Berman says: "The human species is the only species that inhabits the entire earth, that can dive to the bottom of the sea, and fly thousands of feet above the surface of the earth." however, these are all irrelevant activities. All species are unique by definition, otherwise they would not be called a species with its own name. The duck-billed platypus has a unique anatomy and has a unique sense: it finds insects and mollucs buried in sand by detecting their electrical activity. Something humans can not do.
  4. please note that an 'exploration' cannot be falsified, whereas a hypothesis can.
  5. Bernal, personal communication.
  6. Joan Roughgarden (2004) Evolution's Rainbow. Diversity, Gender and Sexuality in Nature and People, p. 258. [ added: 8 June 2004 ]
  7. This contradicts Rensch's Rule, which says that male size relative to female size increases with body size of the species. Rensch's rule has been verified in animals as diverse as arthropods, reptiles, birds and mammals, including primates. In humans this effect seems to be reduced by monogamy, which tends to increase female size relative to male size. See: Malte Andersson and Johan Wallander (2004) Animals behaviour: Relative size in the mating game, Nature 431, 139-141 [9 Sep 2004].
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Further Reading

  • Here is a useful interview with Louis A. Berman.
  • Bruce Bagemihl: Biological Exuberance. Animal Homosexuality and Natural Diversity reviewed on this site.
  • Gender scientists explore a revolution in evolution by Stephanie Chasteen. [ February 19, 2002 ].
  • Carina Dennis (2004) "Brain development: The most important sexual organ", Nature, 427, 390-392. New evidence suggests that the brain begins to develop differently in males and females much earlier than was thought - before sex hormones [testosterone] come into play. Of the 12,000 genes active in the brain, 51 showed different levels of expression in the brains of male and female mouse embryos before the gonads had formed. [ 29 January 2004 ].
  • Sex, drugs and Darwin Letters to the NewScientist magazine about biologist Roughgarden's story. Reader Margaret Monroe suggests that oestrogen mimics in every ecosystem on Earth influence the development of sexual organs, and cause homosexuality. ( However, as we have learned from Berman, the sex organs are normal, the difference is in prenatal brain development. Yet, the environmental explanation could be an alternative for an evolutionary explanation -GK). [9 Feb 2004]
  • Joan Roughgarden (2004) Evolution's Rainbow. Diversity, Gender, and Sexuality in Nature and People. Univ. of California Press, hb 474 pages.
  • The editorial "The complexity of sex. How can a "gay gene" be passed down the generations?" (p.3) and a short news item "Gay genetics" (p.5) in the NewScientist, 16 Oct 2004. Simon LeVay proposes a gene for sexual attraction to men. It makes women to have slightly more children and men [sons] more likely to be gay. This is a new and original explanation for the paradox how a gay gene could be inherited at all and which is also based on data. [ 23 Oct 2004 ]
  • Gregory Cochran An Evolutionary Look at Human Homosexuality. Cochran argues that human male homosexuality is caused by a bug that acts on a specific part of the brain and thereby changes sexual interest. The bug is not HIV. This hypothesis does not say anything about the cause of AIDS, because it is not an explanation of AIDS but of homosexual behaviour. Homosexuality is not caused by HIV (that would be a circularity), but by some other infectious agent and has probably a non-sexual transmission. There is no direct evidence for the hypothesis. 23 Jan 2005
  • Ann Gibbons (2007) 'Hominid Harems: Big Males Competed for Small Australopithecine Females', Science. 30 Nov 2007
    • "Among living people, men are usually bigger than women--but not by much, averaging 5% to 10% larger. Now a study on page 1443 finds that the males of an extinct species of hominid in South Africa took longer to grow up than females--and got much larger. This suggests that these robust australopithecines chose a risky mating strategy: Top males invested energy in bodybuilding in order to possess a harem of females, much like silverback gorillas do today. (...) "Lockwood plans to apply this type of analysis to that and other species to detect when the sexes grew closer in size, a signal that males were investing more in offspring and in longer-term bonds with females. "This is the $64,000 question: When did human dimorphism get smaller?" says Plavcan."
  • David Strah, Susanna Margolis (2004 ) Gay Dads Penguin paperback: An evolution has quietly been occurring in the world of parenting. Recent surveys reveal that millions of children have found loving homes either by being born to, or adopted by, gay men.
  • LGBT parenting refers to lesbian, gay, bisexual, and transgender people parenting children.
  • The Ancient Sexual Revolution that May Have Spurred Human Monogamy, Time, May 29, 2012. Did a sexual revolution, led by low-ranking males and faithful females, lay down the roots of the modern family?
  • Ann Gibbons (2014) 'How we tamed ourselves–and became modern', Science 24 October 2014
    • Some scientists believe that "self-domestication" was key to the evolution of our species. ... lower male aggression, humans are as tame as many of the animals we live with. ... All of these changes tend to make male faces look more like female ones, Franciscus noted at the meeting, and are linked to lower testosterone levels. He and his colleagues proposed that selection for higher levels of social tolerance led to lower levels of testosterone and stress hormones, especially in males, and thus facial feminization."
  • ScienceDaily: Why we have chins: Our chin comes from evolution, not mechanical forces:
    "What happened was this: Modern humans evolved from hunter-gatherer groups that were rather isolated from each other to increasingly cooperative groups that formed social networks across the landscape. These more connected groups appear to have enhanced the degree to which they expressed themselves in art and other symbolic mediums.
    Males in particular became more tranquil during this period, less likely to fight over territory and belongings, and more willing to make alliances, evidenced by exchanging goods and ideas, that benefited each and all.
    The change in attitude was tied to reduced hormone levels, namely testosterone, ...
    and for that to happen, males have to tolerate each other. There had to be more curiosity and inquisitiveness than aggression, and the evidence of that lies in facial architecture." [13 Apr 2015]


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Copyright ©G. Korthof 2003 First published: 22 Dec 2003 Updated: 8 June 2004. F.R: 13 Apr 2015