There’s this gene that about half of all people carry. It’s a pretty nasty gene – it massively increases the risk of the carrier being a murderer or a murder victim, going to jail or dying in an accident. And a new paper in Bioessays – by Prince Henry’s Institute researchers Joohyung Lee and Vincent R. Harley – suggests this gene might be responsible for an aggressive response to sudden stresses.
That could explain the bit about murder, accidents and jail.
Despite all the bad press this gene gets, it isn’t all bad. Carriers have the same number of children as non-carriers. And the biggest winners in our evolutionary history – the people who have left the most descendants – have all been carriers.
The gene is called SRY – short for “sex-determining region on the Y chromosome”. It’s the crucial genetic instruction that triggers an embryo to develop into a male. Without SRY, the embryo becomes a girl.
New findings about the way SRY works might explain the differences in how men and women respond to stress. These findings might also explain why men are more susceptible than women to Parkinson’s Disease, schizophrenia and autism.
Running on adrenaline
The hormone epinephrine achieved fame and brand-name recognition as adrenaline. Everyone from weekend warriors to elite BASE jumpers (is there any other kind?) professes its near-magical capacity to elicit superhuman feats of strength and speed. When the chips are down and the boys need to give 110%, that old adrenaline rush kicks in.
Not only does that adrenaline rush cause many of us to speak in seamless cliché, it prepares us to fight or to flee from whatever threatens us. And that rush actually comes by way not only of adrenaline, but two other hormones in the catecholamine group: norepinephrine and dopamine.
For some decades, the catecholamine-mediated fight-or-flight response was considered the dominant human response to stress. But most detailed studies were done on men. As psychologist Shelley E. Taylor and colleagues from the University of California (Los Angeles) first proposed in a 2000 study, women’s fight-or-flight response isn’t nearly as strong.
Instead, Taylor argued, there is another suite of responses to stress, to which she attached the catchy name “tend-and-befriend". “Tending” involves our nurturing responses – protecting and reassuring ourselves and others. “Befriending” involves creating social networks and seeking out their protection under stress.
Women, according to Taylor, more often respond to stress with this nurturing and social response than with the familiar fight-or-flight. And the urges behind tend-and-befriend behaviours are often regulated by oxytocin – a hormone important in breastfeeding and mother-child bonding – and by female reproductive hormones.
Different strokes for different folks
The discovery that people differ in their response to stress provided a real step forward in the biological study of behaviour. But is the notion that men, when presented with danger, either fight or flee, while womenfolk band together to defend and reassure the nursery herd, any improvement?
To my mind, pop-science simplifications like this play to the outdated nature-nurture dichotomy – the idea that our behaviour is either biological and genetic in origin or that it depends entirely on context, experience and social environment.
Catecholamines stimulate men and women to fight or to flee, even if they differ, on average, in their level of arousal, how readily they fight or how quickly they flee. Likewise, even though oxytocin has long been recognised as an important hormone in women, it also functions in men.
In both sexes, oxytocin facilitates parent-offspring bonding, couple formation and the building of trust. Men tend-and-befriend too, even if they do it on average less often than, and in different ways to women.
The crucial caveat “on average” really counts here. Within each sex, people differ in their tendency to fight-flight or tend-befriend and in how they adopt these broad types of response to stress. And the circumstances of the stress, plus the individual’s life experience, interact with the hormones and hormone receptors to produce a huge variety of behaviours.
Those various behaviours fall – with varying degrees of accuracy – into categories such as tend-and-befriend and fight-or-flight. But the variation within and among individuals is every bit as interesting as the gross average differences observed between sexes.
A new role for SRY
All of this is why a recent press release caught my attention. “Men respond more aggressively than women to stress”, trumpeted the media office of the Bioessays journal, “and it’s all down to a single gene”.
Just as oxytocin and female sex hormones gave Shelley Taylor the female-centric basis for tend-and-befriend, the new paper by Lee and Harley suggested the male bias in fight-or-flight might be due to SRY.
The idea flows from some exciting new research on the SRY gene. For over 20 years we have known that SRY determines sex by making a protein (called, surprisingly, SRY protein) that stimulates a few key embryonic cells to develop into testes.
Testosterone from the testes then signals other tissues to make the body into a male. Without SRY protein, the body follows the default settings and those same embryonic cells become ovaries. For some time, researchers thought that once SRY had initiated testes development its work was done.
In recent years, however, various researchers, including Harley, have found SRY protein in the human brain, and other tissues. And in those places it interacts with catecholamines such as adrenaline and dopamine.
Lee and Harley’s new paper suggests SRY protein could make male tissues more susceptible than female tissues (which lack SRY protein) to the fight-or-flight symptoms of increased nervous activity, circulation and capacity to move rapidly.
For too long, explanations of male-biased behaviour have looked no further than testosterone. Men tend to have about ten times as much circulating testosterone as women, and sex differences are often casually written off to differences in testosterone concentration.
Excitingly, Lee and Harley make the very testable suggestion that an exclusively male gene – known to be involved in catecholamine signalling – might make men more prone to the fight-or-flight response than women.
Unfortunately – and I’m not too surprised given the breathless press release – the interesting and important implications of this finding have been largely overlooked by a news media hungry for simple, familiar stories.
According to one report “the macho gene that makes men behave aggressively has been found”. Let’s leave aside the fact that SRY was identified in 1985, and the quite obvious fact that because SRY makes men male it is as responsible as any gene can be for differences between males and females.
But reports such as the above simply stoke the idea that complex social issues – such as sex differences in behaviour – can be entirely understood with the identification of a gene involved.
Once we know, as we do, that men are many times more likely than women to be murderers and murder victims, go to jail or to suffer from certain diseases, our work has barely even begun. These facts require both explanations and an understanding of how they come to be before they can become anything more than interesting observations.
What excites me most about Lee and Harley’s paper is that it suggests new pathways for research into male-specific disorders. Parkinson’s Disease, autism, schizophrenia and attention-deficit/hyperactivity disorder are all more common in men than in women. And they all involve altered catecholamine levels.
In Parkinson’s Disease, for example, nerve cells sensitive to dopamine are progressively lost. Could the presence of SRY somehow make male nerve cells more vulnerable to death? Perhaps doctors could use this knowledge to minimise the harmful consequences of SRY protein in the brain.
To me, understanding how evolution has generated differences in men’s and women’s susceptibility to appalling diseases is so much more interesting and important than another round of titillating stereotypes about badly behaved men and über-nurturing women.
You can follow Rob Brooks on Twitter @Brooks_Rob.