New Scientist - 08.02.2020

(Wang) #1
8 February 2020 | New Scientist | 23

JOSIE FORD


Comment


Views


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.

The columnist
Graham Lawton on
why aiming for net
zero could work p24

Letters
Ask an AI to explain
how an AI made a
decision p26

Aperture
A swarm of locusts
flies through Kenya
eating crops p28

Culture
How to fight back
against everyday
racism p30

Culture columnist
Sally Adee on sci-fi
that turns our cities
“smart” p32

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
Free download pdf