New Scientist - 08.02.2020

8 February 2020 | New Scientist | 23

JOSIE FORD

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Andrew Barron is a

neuroethologist at

Macquarie University

in Sydney, Australia

H

OW did human same-sex

attraction come to be?

At first glance it seems to

be an evolutionary paradox. For a

trait to evolve, it has to be passed

on to children to whom it confers

some sort of advantage. But as

gay sex, of itself, cannot yield

offspring, we should expect

same-sex attraction to go extinct.

Evolutionary biologists have

long struggled with this paradox,

but my colleagues and I believe

that if you come to the puzzle from

a different angle, the apparent

contradiction disappears.

The trick is to recognise the

complexity of human sexual

activity and sexuality.

Firstly, same-sex attraction only

looks like a paradox if we consider

human populations to be made up

of two distinct groups: people who

are exclusively gay and people

who are exclusively straight. But

human sexuality isn’t like this.

Every study since the

pioneering work of Alfred Kinsey

and his colleagues in the 1940s

and 50s has backed up the idea

that sexuality varies continuously

from a majority of people who

identify as exclusively straight to a

minority of people who identify as

exclusively gay. In the middle are a

range of people, including those

who identify as bisexual, mostly

straight or mostly gay.

Acknowledging this spectrum

radically changes the evolutionary

question. It means that we should

be asking how variation in

sexuality evolved, not just how

same-sex attraction has evolved.

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Secondly, the majority of sex,

be it gay or straight, isn’t for

reproduction. For humans and our

chimpanzee and bonobo relatives,

sex has a range of social functions

that include play, social bonding,

affiliation and even barter, conflict

resolution, dominance and

appeasement. Thinking about the

evolution of sex has to consider

these social functions as well.

My colleague Brian Hare at

Duke University and I proposed a

new hypothesis for the evolution

of same-sex attraction in a recent

paper published in Frontiers in

Psychology (doi.org/dk2x).

An important aspect of recent

human evolution has been

selection to be proactively social:

prosociality. If survival and

reproduction depend on being

part of a functional social group

(as was the case for early humans

and our primate ancestors),

individuals that are highly

prosocial can rapidly integrate

into a group, operate better within

a group and have greater mobility

between groups. Selection for

prosociality has resulted in a

whole range of traits for greater

social awareness and tolerance,

better social communication and

reduced aggression. This includes

an expanded role for sex in social

contexts that include adult social

bonding, adult play and conflict

resolution. That applies to both

gay and straight sex.

Consider bonobos, our closest

primate relative. They diverged

from chimpanzees about 2 million

years ago and as a species are

extremely prosocial. They are also,

famously, highly sexual, using sex

(gay and straight) for a range of

social functions. We think

something similar occurred in

recent human evolution. Selection

for increased prosociality

increased the frequency and

diversity of expression of social

sex and increased same-sex

attractions.

A recent study showed human

sexuality could be influenced by

hundreds of genes and there is no

single gene for same-sex attraction.

Instead, many genes influence

variation in sexuality, each acting

in small and different ways, which

may explain the variation we see

in people’s sexuality.

Variation is expected for

complex traits involving hundreds

of genes: consider human IQ or

height, for example. But we don’t

feel a need for special evolutionary

arguments to explain the

existence of very smart or very tall

people, they are just one end of

spectra of variation. ❚

The evolution of sexuality

Same-sex attraction isn’t an evolutionary paradox – our explanations

for how it came to be are just wrong, says Andrew Barron