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Homosexual males, their brains and evolutionThe 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.
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
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?
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.
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.
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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|Copyright ©G. Korthof 2003||First published: 22 Dec 2003||Updated: 8 June 2004. F.R: 13 Apr 2015|