Newborn Behavioral Differences: Cambridge Study Examines Early Sex Disparities

It is one of the most enduring and debated questions in science and philosophy: How much of the behavioral difference between men and women is shaped by biology, and how much is forged by our environment?

A new study conducted by researchers from the University of Cambridge is attempting to shed fresh light on this age-old nature-versus-nurture debate. The research reveals that subtle behavioral differences between newborn boys and girls may begin to manifest merely hours after birth, offering a fascinating glimpse into the earliest moments of human development.

According to the research team, these early divergences could be driven by pre-natal forces, such as differential exposure to sex hormone levels, or they might reflect a slightly accelerated pace of cognitive development among newborn girls compared to boys.

To explore these possibilities, the Cambridge researchers designed an experiment involving newborns who were, on average, just 33 hours old. The infants were presented with two distinct videos: one displaying a human face, and the other featuring an animated Newton’s cradle, which consists of swinging metallic spheres designed to represent a dynamic, non-social, mechanical motion.

When analyzing the infants’ visual attention, the researchers observed that both sexes spent slightly more cumulative time looking at the human face than at the swinging metallic spheres of the Newton’s cradle. However, a closer examination of the data revealed a notable distinction. Girls spent proportionally more of their total looking time focused on the human face than boys did.

Quantifying the absolute time spent looking, researchers found that newborn girls stared at the face for approximately 10 seconds longer than they did at the Newton’s cradle. In contrast, the boys did not exhibit a statistically significant difference in the amount of time they devoted to watching the face versus the mechanical toy. Based on these findings, the study authors concluded that the newborn girls demonstrated a clear preference for the human face, whereas the boys showed no discernible preference between the two visual stimuli.

What Can We Learn from This Research?

It can be tempting to interpret these findings as proof that girls are naturally more social from the very start of life, possessing an innate, biological drive to connect and interact with other human beings. Unfortunately, child development experts caution that human behavior and psychology are rarely so simple.

Because the study did not follow the participating newborns as they grew older, the research cannot determine whether these early visual preferences have any predictive value for later behavior. Consequently, scientists can only state that at roughly 33 hours of age, the girls in this specific experiment displayed a stronger preference for a human face than the boys did.

Furthermore, this new investigation enters a scientific landscape where research surrounding sex differences at birth remains mixed and inconclusive. A comprehensive 2025 review of existing literature on infant behavior found no widespread, definitive consensus regarding significant differences between newborn boys and girls when it comes to their fixation on human faces.

That same 2025 review also examined other putative early markers of empathy and social engagement, such as "contagious crying"—the phenomenon where an infant cries reflexively in response to the sound of another infant crying. Researchers found no reliable differences between newborn boys and girls in their propensity for contagious crying, further underscoring how complex and nuanced early behavioral development truly is.

Despite these caveats, the authors of the Cambridge study suggest that the sex differences observed in their experiment point toward biological factors that are already active well before birth. Prenatal hormone environments, particularly fluctuating levels of sex hormones, are prime candidates for contributing to early behavioral divergences.

Researchers point out, for example, that male fetuses experience a distinct surge of testosterone during early pregnancy. The volume of this surge is estimated to be about two and a half times higher than the levels to which female fetuses are exposed. This powerful hormonal bath plays a foundational role not only in shaping physical development in utero, but also in influencing the trajectory of brain development.

At the same time, experts urge observers not to entirely discount the subtle influence of gender socialization—the process by which infants begin absorbing societal expectations regarding their roles as boys or girls. Although researchers emphasize that socialization within the first few days of life is likely minimal, they acknowledge that adults may unconsciously alter how they look at, speak to, and physically handle newborn boys versus newborn girls, introducing micro-environments that could theoretically impact infant responses.

Another compelling hypothesis involves the natural timeline of human maturation. Clinical observations have frequently suggested that girls, on average, display slightly faster cognitive and physiological development throughout childhood. Even during the neonatal period, subtle maturational discrepancies between the sexes have been documented in clinical literature. The study’s authors speculate that this slightly accelerated maturation rate could equip newborn girls to process complex visual information, such as the features and movements of a human face, with greater efficiency than their male peers.

Finally, researchers emphasize the need for caution when evaluating the visual stimuli used in the experiment. The video of the human face inherently possessed different colors, motion dynamics, and compositional elements than the digital animation of the Newton’s cradle. Although the research team made diligent efforts to match several of these baseline visual characteristics across both videos, they cannot entirely rule out the possibility that a specific visual feature—rather than the social versus non-social nature of the content itself—helped drive the differences in how long the infants chose to look.