A new study of infants only hours old has uncovered measurable differences in how baby girls and boys direct their attention, offering fresh evidence that some behavioural distinctions between the sexes may originate in biology rather than culture or upbringing.
When newborns aged between eight hours and three days watched side-by-side videos on a laptop screen, girls allocated a greater proportion of their looking time to a woman's face compared with boys, who distributed their gaze more evenly between the social and nonsocial stimuli. The finding emerges from research conducted at the Rosie hospital in Cambridge with 130 infants, and has been published in the journal Biology of Sex Differences.
The question of how much human behaviour stems from innate biology versus environmental shaping has long fascinated and divided researchers. Because the participating babies were tested within their first few days of life, the researchers argue that cultural conditioning or learned social experience could not account for the observed pattern. This points instead toward biological mechanisms—chiefly genetics and prenatal hormone exposure—as the driving force.
What did the study show?
Infants were settled on a parent or carer's lap while an eye-tracking system recorded their gaze as they viewed two videos simultaneously. One displayed a young woman's face with natural movement; the other showed metallic balls swinging in a Newton's cradle pattern. Both sexes demonstrated an overall preference for the human face over the mechanical object. However, girls devoted a significantly larger share of their total looking time to the face, whereas boys showed no clear preference between the two stimuli when absolute looking time was measured.
According to Medical Xpress, the key distinction was not in how long infants looked overall, but in the proportion of gaze time allocated to faces versus objects. Female newborns exhibited a genuine preference for the social stimulus, while male newborns showed no measurable preference between the two.
Yumnah T. Khan, a PhD student at the University of Cambridge Autism Research Centre who led the work, emphasised the significance of the timing.
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? What our research shows is that at least some differences are present at a small scale very early on in life.
What do these early differences mean for development?
The researchers stress that their findings describe average patterns across groups of boys and girls rather than universal traits. Substantial variation and overlap existed between the sexes, meaning the results do not apply uniformly to all children. Nevertheless, early-emerging differences in social attention may help explain some well-documented developmental trends observed later in childhood.
Girls typically acquire language skills and certain social competencies earlier than boys on average. If female infants begin life with heightened attentiveness to faces—a crucial source of social and linguistic information—this biological starting point could facilitate the development of these abilities over time.
If they're showing heightened social interest from birth onwards, that might facilitate the development of these skills,Khan noted, though she acknowledged this remains a hypothesis requiring further investigation.
The Cambridge team plans to follow participating infants into toddlerhood to test whether early attention patterns predict later developmental outcomes. According to the University of Cambridge Autism Research Centre, infants enrolled in this research line are typically reassessed at 18 months of age to measure developmental progress and other characteristics.
Could this relate to mental health differences?
A broader question animating the Cambridge research concerns the roots of sex-based disparities in neurodevelopmental and psychiatric conditions. Autism and attention-deficit hyperactivity disorder (ADHD) are diagnosed more frequently in males, while anxiety disorders and depression show higher prevalence in females. Whether subtle neurobiological differences present at birth contribute to these patterns remains unclear, but Khan and her colleagues are investigating the possibility.
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.
What biological mechanisms might explain the pattern?
Prior research has identified similar sex-based differences in newborn attention and social perception. These variations are thought to stem from hormonal influences during pregnancy, particularly testosterone exposure in the womb. Male foetuses are exposed to more than twice the testosterone levels of female foetuses during the first and second trimesters, a difference that may shape the developing brain's structure and function.
The prenatal hormone environment has long been implicated in shaping sex-typical patterns of behaviour and cognition. This study adds to accumulating evidence that such biological influences may be detectable within hours of birth, before postnatal social learning has had meaningful opportunity to take hold.
What happens next?
The Cambridge Baby Study, of which this research forms part, continues to recruit newborns from postnatal wards during their first few days of life. According to the NHS research summary, the replication-and-extension study uses gaze-tracking technology to investigate whether newborn social attention differs by sex and whether early attention preferences relate to parental characteristics and later developmental trajectories. Researchers will continue following participants through infancy and into toddlerhood to determine whether the early patterns observed in the laboratory predict meaningful differences in development and behaviour as children grow.
The full findings appear in Biology of Sex Differences under the title "Sex differences in face looking preferences at birth" (DOI 10.1186/s13293-026-00976-9).
