I have had this on my computer for 6 months now, but i can’t get myself to work on it. Basically just need to insert some references and look thru some studies. Perhaps the theory is bunk. I did try to find data on kinsey scale ratings vs. number of children, but didnt find anything.

Since it is of not much use to me lying on my hard drive, i will publish it now.

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An evolutionary theory of the origin of same-sex sexual acts based on social bonding

Emil Kirkegaard, Department of Linguistics, Aarhus University

Introduction

Homsexuality in humans and other animals [get some refs from wikipedia] is a mystery [ref to Buss’s textbook]. The mystery goes like this: homosexualiy seems so obviously bad for inclusive fitness that it very strange that evolution has not filtered it out. The reason that it is bad, is that homosexuals do not have children. This is not correct [insert studies with data about sexuality and number of children], and the misleadingly simple terms “homosexual”, “bisexual” and “heterosexual” make it harder. It is better to use a Kinsey scale of sexuality [cites, perhaps grab some from Wiki]. Using such a scale, one would expect that people that score more towards the homosexual end have fewer children. [look for studies about this]. But people with scores toward the low end of the Kinsey scale are here and data from research on people’s ability to detect people that are ‘homosexual’ (popularly called the gaydar) suggests that humans have evolved the ability to pretty reliably detect people that have sexual interest in the same sex as themselves. [cite some of the studies mentioned on Wikipedia].

In this paper we propose an evolutionary explanation of sexual interest in the same sex. This theory is one among many theories that attempt to explain the evolutionary origins of same-sex sex acts. Some of these theories work via kin selection, and are called ‘good uncle’ theories. The idea is that even though a ‘homosexual’ family member may have reduced fitness, he conves fitness indirectly by helping his relatives. Another interesting theory is that same-sex sexual behavior is due to sexual antagonism which is when genes that improve fitness in the one sex, reduce it in the other and but are positive on balance. Such genes can be selected for and a recent study by [mentioned on Wikipedia page] supports this theory. [use Wikipedia for more cites]

The theory

The theory is based on three assumptions:

1. Having an interest in both sexes is good for fitness, i.e., scoring somewhere between 1-5 on the scale

2. Sexual preferences follow a normal distribution or a skewed Gaussian distribution that is skewed towards the heterosexual end of the Kinsey scale

3. Exclusive homosexuality is bad, i.e. scoring 6 on the scale

The reasoning for these hypotheses is as following.

1. Having an interest in both sexes is good for fitness, i.e., scoring somewhere between 1-5 on the scale

The support is based on social bonding betwen same-sex people. The idea is that having sex helps create bonds between two animals, including humans. This idea works for humans and non-human animals alike. It is known from animal sexuality research that males tend to engage in same-sex sex acts for bonding purposes [cites]. Having stronger bonding with members of the same sex is potentially useful for (surviving) warfare and social hierarchy purposes. These things have over evolutionary time been more important for the human male. So, this effect is stronger for males and would thus skew the distribution more towards homosexuality for men more than for women. This is indeed the case. There are more men than women that rate themselves 6 on the scale. [cites and illustrations for each sex].

Testing this hypothesis in industrialized humans seems hard as there is no longer much of a struggle for survival (natural selection is almost completely without force). In fact, we are probably living in the least violent time ever in evolutionary history (Pinker 2011). But it should be possible to test it in the surviving traditional societies spread around the world. One way to do that is to do a longditutional study of the survival rates for people and their Kinsey scale ratings. If the hypothesis is correct, there should be a slight positive effect on survival rates for the people that are more bisexual, especially for men. This effect may not appear if the society is very hostile to same-sex sexual acts. For this reason, one would need a larger sample than one society to disprove the hypothesis. We are not aware of any studies that have done this [look for such studies].

2. Sexual preferences follow a normal distribution or a skewed Gaussian distribution that is skewed towards the heterosexual end of the Kinsey scale

For this theory to work, sexuality has to be either normally distributed or skewed to the one end. If there were more people at the extreme ends of the scale, this theory would not work. But sexuality does follow a skewed Gaussian distribution. [cites and illustration]

3. Exclusive homosexuality is bad, i.e. scoring 6 on the scale

This is the usual mystery. People that score towards the homosexual end of the scale have fewer (non-adopted) children than those that are in the middle or towards the heterosexual end. [need data but i think this is true]

Selection preasures

The selection preasure for bisexuality, i.e., scale 1-5 moves the sexuality average towards the middle area, while the selection against exclusive homosexuality, i.e. scale 6 moves the sexuality average away from 6. If the theory is correct, then these preasures move the sexuality average to some midway between 0 (furtherst away from 6) and 1-5 (whatever is optimal). Without data about how strong the hypothesized extra strength of bonding with same sex persons is worth, there is no way to deduce from this theory where on the scale the optimal point is for bisexuality. Perhaps the occassional same-sex sexual act is strong enough to maximize the effect (this would favor scale 1-2), perhaps more regular or very regular same-sex acts are necessary (this would favor scale 3-5).

Conclusion

We have proposed a theory to explain the origin of homosexual behavior (exclusive or not) in humans and other animals. Some research seems to support it but more decisive empirical tests are necessary, including ones in traditional societies.

References

Pinker, Steven (2011). The Better Angels of Our Nature: Why Violence Has Declined. Viking Adult.