Girls and Boys Begin Responding to the World Differently Just Hours After Birth

The question of how much of the behavioral differences between men and women is shaped by nature and how much by upbringing and social environment has long been one of science’s enduring debates. A new study may add some intriguing details to the discussion: researchers have found differences in the responses of newborn boys and girls that emerged as early as around 33 hours after birth.

Researchers at the University of Cambridge suggest that factors acting before birth may play a role, particularly levels of sex hormones. Another possibility, they say, is that the differences may be linked to slightly faster cognitive maturation in girls during the first days of life.

In the experiment, newborns who were around 33 hours old were shown two videos. One featured a human face, while the other showed an animated Newton’s cradle, in which metal balls successively collide with one another.

Both boys and girls watched the face slightly longer than the Newton’s cradle. However, the proportion of time girls spent looking at the face was noticeably higher than among boys.

In absolute terms, girls spent about 10 seconds longer watching the face than the toy. Among boys, researchers found no significant difference between the amount of time they spent watching the two videos.

According to the scientists, this may suggest that girls showed a certain preference for human faces within their first hours of life, while boys did not display a pronounced preference.

However, the researchers stress that these findings cannot be taken to mean that girls are naturally more sociable or are born with a stronger interest in interacting with people.

Nevertheless, the authors believe the results may point to certain factors that operate before birth. In particular, they suggest that sex hormone levels could influence the development of behavioral differences.

According to the researchers, boys are exposed to approximately 2.5 times higher levels of testosterone than girls early in pregnancy. This hormone is involved not only in physical development but also in brain development.

At the same time, the scientists urge caution in considering the influence of the social environment. Even in the first days of life, babies begin encountering ideas about what it means to be a boy or a girl. The researchers believe the impact of gender socialization is probably minimal at this age, but they cannot rule it out entirely.

Adults may, for example, look at boys and girls slightly differently, speak to them differently or touch them in different ways — and such subtle variations could theoretically influence infants’ behavior.

There is another possible explanation as well. On average, girls may show slightly faster cognitive development during childhood. Some studies have also pointed to small differences in brain maturation already among newborns. The researchers suggest that faster maturation could allow girls to process complex visual stimuli, such as human faces, more effectively.

However, the study has an important limitation. The video of the face and the Newton’s cradle differed noticeably in color, movement and composition. The researchers attempted to match these characteristics as closely as possible, but they could not completely rule out the possibility that the infants’ responses were influenced by visual features of the videos themselves rather than by their social or nonsocial content.