It’s an age-old question: How much of the differences between the way men and women behave are a result of nature, and how much is nurture?
A new study could shed more light on this. It shows different behavior between newborn boys and girls could appear just hours after birth.
Researchers from the University of Cambridge believe forces at work before birth — like sex hormone levels, might play a role, or even that the changes are a result of a slightly faster cognitive development in newborn girls.
They showed newborns — only about 33 hours old — two videos. One was a human face, the other, an animated Newton’s cradle (swinging metallic spheres).
Both sexes spent slightly longer looking at the face than at the Newton’s cradle. But researchers found that girls spent proportionally more of their looking time on the face than boys did.
In terms of absolute time spent looking, girls looked about 10 seconds longer at the face compared to the Newton’s cradle, while the boys didn’t have a significant difference between the time spent looking at either video.
The researchers say this suggests girls had a preference for the face, while the boys didn’t show a preference.
What can we learn from this research?
You might assume this means that girls are naturally more social, with an innate interest to be with and interact with other humans. Unfortunately, it’s not that simple.
This study didn’t follow the newborns as they grew, so it can’t tell us whether these early differences predict later behavior. We can only say that, at around 33 hours old, girls in this experiment showed a stronger preference for the face than boys did.
It’s also important to note that this study joins a body of research that doesn’t tell a clear story about sex differences at birth. A 2025 review of research on this subject found that there was no significant difference between boys’ and girls’ fixation on human faces.
They also found there wasn’t any difference in boys and girls when it came to “contagious crying” — crying in response to another infant crying.
Researchers say differences between sexes observed in this new study could point to factors at play before birth, like sex hormone levels, could be contributing to sex differences in behavior.
They note that boys get a surge of testosterone in early pregnancy — about 2.5 times what girls are exposed to, which plays a role in physical development as well as brain development.
Researchers also urged readers to consider gender socialization — learning about their role in the world through whether they’re a girl or a boy — even in the first few days of life. Though they say this is “likely minimal,” there might be subtle differences in they way adults might look at, speak to, and touch newborn boys vs. girls.
Another possibility is that girls, on average, show faster cognitive development during childhood. Even in newborns, subtle maturational differences between the sexes have also been reported. The researchers speculate that faster maturation could allow newborn girls to more efficiently process complex visuals, like faces.
There’s also the important note that the video of the face was very different in color, motion, composition from Newton’s cradle. Although the researchers tried to match some of these characteristics, they can’t rule out the possibility that some visual feature of the videos—not their social versus nonsocial content—helped drive the difference.
