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The question dates back to the first man and woman: how does the world shape the behaviour of the sexes, and what differences, if any, are present at birth?
Untangling the contributions of nature versus nurture has proved challenging for scientists, and controversial at times, but a new study of babies sheds light on at least one aspect of the puzzle.
When newborns were played side-by-side videos of a face and a toy, girls spent proportionally more time, on average, watching the face than boys did, suggesting that differences in attention may exist when we are born.
Because the babies were only a day or so old it is unlikely the effect was driven by culture or social experience. That leaves biological factors such as genetics and exposure to different levels of testosterone in the womb as the leading suspects.
“This is a very active and heated debate in the field of sex differences: what are the causes behind sex differences and how early in life are they present?” said Yumnah Khan, a PhD student at the Autism Research Centre at the University of Cambridge. “What our research shows is that at least some differences are present at a small scale very early on in life.”
For the study, babies aged from eight hours to three days old watched a laptop while settled on their parent or carer’s lap. The screen showed side-by-side videos with a young woman’s face on one side and a Newton’s cradle toy on the other. An eye tracker monitored where the babies looked as the video played.
Both sexes showed more interest in the video of the woman’s face than the clip of the Newton’s cradle swinging back and forth. But the girls favoured the face over the toy more than the boys did. Details are published in the journal Biology of Sex Differences.
The findings, based on tests with 130 babies at the Rosie hospital in Cambridge, reveal average differences between groups of boys and girls; they don’t apply to all girls and all boys. There was still plenty of variation and overlap between each sex, the researchers said.
Small differences at birth are unlikely to define how children learn and develop, but they may play a role in some long-observed trends. On average, girls learn to talk earlier than boys, for example, and develop certain social skills at a younger age, too. “If they’re showing heightened social interest from birth onwards, that might facilitate the development of these skills,” said Khan, though this is only a hypothesis. Support for the idea could come from further studies that follow the babies until they are toddlers.
Another question the Cambridge team is exploring is whether subtle differences between the sexes at birth have any bearing on people’s vulnerability to neurodevelopmental and psychiatric conditions. For example, autism and ADHD are more commonly diagnosed in males, while anxiety and depression are more common in females. “That’s the root issue that we’re really trying to understand here when we’re asking are there any differences present in males and females early in life and what causes these differences,” Khan said.
Previous work has found similar on-average sex differences in brain structure at birth. These are thought to be driven by hormones in the womb, with male foetuses exposed to more than twice the level of testosterone as females in the first and second trimesters.
