Can brain biology cause behavioural differences between – as well as among – men and women?
New research – which involved recruiting twins, analysing genetic data and considering hormones, environment and culture – has found there are differences between the brain and behaviour between men and women, but it’s the differences between people within a sex which show the greater variation.
Queensland Brain Institute researcher Associate Professor Margie Wright and Associate Professor Brendan Zietsch from The University of Queensland’s School of Psychology answer the big questions resulting from the research.
Q: What have you found?
Brendan Zietsch: We found that there is a small link between differences in the brain and behaviour between men and women. The differences tend to be in cognitive abilities, such as spatial and verbal abilities, as well as personality traits.
Q: What are the behavioural differences between men and women?
Brendan Zietsch: On average, men have higher scores in arithmetic, spatial and information processing. Women are more verbally fluent, extraverted, open and agreeable.
Q: Do all men and all women have strengths in these respective areas?
Margie Wright: No, they don’t. Some men will develop a more female-like brain, while other men will develop an exaggeratedly male-like brain. It’s the same for women. They’re not all the same.
Q: What do we know about differences among men – and among women?
Margie Wright: The bottom line is: Yes, there are small but consistent differences in brain and behaviour between males and females when you look at the total population, but these are dwarfed by the differences between people within a sex. The individual differences are much larger within sex groups than between the sexes.
Q: What are the reasons for variation between people of the same sex?
Margie Wright: Genetic predispositions – as well as exposure and sensitivity to sex hormones – cause that variation among the brains of both sexes.
Q: Physically, how are the brains of men and women different?
Brendan Zietsch: There are various differences in the regions of male and female brains, both in terms of size, shape and structure. Some regions that differ include the amygdala – that helps define and regulate emotions; the pallidum – that is involved in rewards and motivation; the putamen – that aids in movement of the limbs; and the nucleus accumbens – involved in goal-directed behaviour.
Q: Is brain structure the main factor that explains differences in behaviour between men and women?
Brendan Zietsch: No, the research didn’t show that. The fact that the links were small suggests that brain structure is only one of many factors explaining behavioural sex differences.
Q: Are there also environmental factors that can account for differences in behaviour between the sexes – as well as among people of the same sex?
Brendan Zietsch: Yes, there are. It’s one of the reasons we used twins and their siblings in the research. We wanted to estimate the extent to which individual differences in brain and behaviour can be explained by genetic and environmental influences.
Q: How have you done that?
Brendan Zietsch: We analysed two large independent imaging data sets – one with 1,040 people from 616 families and another with 1,113 people from 428 families. We found that even genetically identical twins differ in their brains and behaviour, though the differences are less than for non-identical twins or normal siblings.
Q: What does that mean?
Brendan Zietsch: It means that the brain and behaviour differences between the sexes are similarly influenced by both genes and environment.
Q: How do these findings contribute to ongoing research?
Brendan Zietsch: We have trained an algorithm – essentially a set of computer instructions – to study MRI scans and recognise the differences in brain shape or structure and accurately predict sex on that basis. That takes away human bias and tells us that the differences – though small – are clear. It also means other researchers will be able to replicate that and use it to advance their own studies.
Q: Where will the research take you next?
Brendan Zietsch: Future research with even larger numbers of people and measures, and participants at different developmental stages, will help us further understand the biological and social influences on brain and behavioural sex differences. That will help to answer questions such as at what stage of our lives sex hormones play the most prominent role in influencing brain and behaviour – and whether specific sex hormones have distinct influences on brain and behaviour. It will also shed light on the distinction between biological sex and gender differences.
Q: Can the research help build an understanding of the human brain, other than in behavioural differences between the sexes?
Brendan Zietsch: The algorithm opens the door to a greater understanding of brain biology linked to neurodevelopmental conditions such as autism.
The research involved Dr Liza van Eijk (now at James Cook University) and has been published by the Association for Psychological Science (https://doi.org/10.1177%2F0956797621996664)
Media: Erik de Wit, QBI Communications, e.dewit@uq.edu.au, +61 447 305 97