Newborn Behavior Study Hints at Early Sex Differences, Re-igniting the Nature vs. Nurture Debate

It remains one of the most enduring and debated questions in human science: To what extent are the behavioral differences between men and women shaped by biology, and how much are they sculpted by the environment and upbringing?

A new study published by researchers from the University of Cambridge is adding fresh data to this age-old dialogue, suggesting that behavioral variances between newborn boys and girls may begin to manifest mere hours after birth. The findings point toward potential prenatal influences, such as sex hormone exposure, or subtle variations in early cognitive maturation between the sexes, though scientists involved in the project emphasize that the implications remain complex and far from definitive.

To explore this foundational question, the research team observed newborns with an average age of just 33 hours. The infants were presented with two distinct visual stimuli: a recording of a human face and an animation of a Newton’s cradle, featuring swinging metallic spheres.

Both sexes ultimately spent slightly more cumulative time looking at the human face than at the mechanical toy. However, a deeper analysis of the data revealed a distinct behavioral divergence. The newborn girls spent a proportionally greater share of their total viewing time focused on the face compared to the boys.

In terms of absolute time, the female infants looked at the face for approximately 10 seconds longer than they did at the Newton’s cradle. Conversely, the male infants displayed no statistically significant difference in the amount of time they devoted to either video, indicating a lack of a clear preference between the two stimuli. According to the research team, this suggests that newborn girls exhibited a measurable preference for the human face, whereas the boys showed no such inclination at this stage of life.

What Can We Learn from This Research?

It can be tempting to interpret these results as evidence that females are hardwired from birth to be more social, possessing an innate and instinctive drive to interact with other human beings. However, developmental psychologists and researchers caution that drawing such a definitive conclusion oversimplifies a vastly intricate process.

Because the study did not track the participating infants as they grew older, it cannot establish whether these early visual preferences serve as reliable predictors of later behavior or adult personality traits. The observation is strictly bounded by the developmental window in which it was measured: at approximately 33 hours of age, the girls in this specific controlled experiment demonstrated a stronger focus on a human face than the boys did.

Furthermore, this new investigation enters a scientific landscape where research into sex differences at birth has yielded mixed and sometimes contradictory results. A comprehensive review published in 2025 examining existing literature on infant behavior found no consistent or significant difference between newborn boys and girls when measuring their fixation on human faces.

That same 2025 review also evaluated other early social responses, such as "contagious crying"—the phenomenon where an infant cries in reaction to the sound of another baby crying—and found no measurable disparity between the sexes. Given these broader inconsistencies, scientists argue that individual studies must be viewed as pieces of a larger, still-incomplete puzzle rather than definitive proof of innate psychological divides.

Nevertheless, the authors of the Cambridge study point to several biological and environmental factors that could account for the differences observed in their laboratory setting. Among the leading biological hypotheses are prenatal influences, including the distinct hormonal environments experienced by male and female fetuses in utero.

Researchers note that male fetuses experience a substantial surge of testosterone during early pregnancy, reaching levels roughly 2.5 times higher than those to which female fetuses are typically exposed. This hormonal bath plays a critical role not only in the physical differentiation of the body but also in the structural and functional development of the brain, leading some scientists to theorize that it could subtly influence early sensory processing and behavioral tendencies.

At the same time, the study’s authors advise considering the potential impact of early gender socialization—the process by which infants begin to learn their societal roles based on whether they are identified as girls or boys. While the researchers emphasize that any socialization occurring within the first few hours of life is likely to be minimal, they acknowledge that subtle, unconscious differences may exist in how adults look at, speak to, and handle newborn boys versus newborn girls, even in a hospital nursery setting.

Another compelling possibility involves the rate of early neurological maturation. Clinical observations have frequently suggested that girls, on average, demonstrate slightly faster cognitive and physiological development during early childhood. Even in the immediate newborn period, subtle maturational differences between the sexes have occasionally been documented. The researchers speculate that a marginally more advanced state of neurological maturation could enable newborn girls to process complex visual stimuli, such as a human face, with greater efficiency or sustained attention.

At the same time, the study’s authors remain transparent about the methodological limitations of their experiment. The video of the human face and the animation of the Newton’s cradle differed significantly across multiple dimensions, including color, motion profiles, and visual composition. Although the researchers made concerted efforts to match certain visual characteristics between the two clips, they cannot entirely rule out the possibility that a specific visual feature of the videos—independent of whether the content was social or mechanical—helped drive the divergent viewing patterns observed in the data.