Some babies seem drawn to faces from the start, while others are just as happy staring at a toy. A Swedish twin study suggests that a good part of that difference is inherited: at five months, how long an infant keeps looking at a face rather than at objects is substantially shaped by genes.
The study was led by Ana Maria Portugal and Terje Falck-Ytter of Uppsala University and the Karolinska Institutet, with colleagues at King’s College London and the University of Surrey. It was published in Nature Human Behaviour on November 27, 2023, and followed a pre-registered analysis plan.
A face among four objects
The researchers drew on the Babytwins Study Sweden, a community sample of same-sex twins. A total of 536 infants, about 168 days old on average, watched screens showing five items at once: one face and four non-social objects. An eye tracker recorded two things. Face orienting was how often the baby’s first look went to the face, which is thought to reflect fast, subcortical processing. Face preference was the share of looking time spent on the face, thought to reflect emerging attention control.
With one face among five items, chance would be 20%. Babies beat that easily. On average their first look went to the face on 30% of trials, and they spent 44% of their looking time on it.
Because identical twins share all of their segregating genes and fraternal twins about half, comparing how alike the two kinds of pairs are lets researchers split the variation into genetic and environmental parts.
Genes matter, the shared home does not show up
For sustained face preference, identical twins correlated at 0.46 and fraternal twins at 0.21. The best-fitting model put the heritability of face preference at 0.46 and of face orienting at 0.19. The rest was explained by experiences unique to each twin, including measurement error. There was no evidence for an effect of the shared environment, such as family circumstances, and no sex difference.
The two measures were moderately linked, and part of the genetic influence on face preference was shared with orienting. A third measure, how many of the five objects a baby explored in the first ten seconds, showed no genetic influence at all.
Earlier work in the same twins found that whether infants look more at the eyes or the mouth is also heritable. The new analysis found the two traits were only weakly related and driven by independent genetic factors.
Small links to later language
The team also checked polygenic scores for autism, ADHD, schizophrenia, depression and bipolar disorder. None was associated with either face measure.
Follow-up questionnaires from parents at 14 and 24 months found mostly null results. The one exception: babies with a stronger face preference at five months understood somewhat more words at 14 months, according to parents, an effect that explained under 2% of the variation.
The authors say the findings show that differences in “selection of perceptual input” are already heritable at five months. In practice that means part of what a baby learns from the people around it may depend on where its genes lead its eyes in the first place.
Study Details:
- Title: Infants’ looking preferences for social versus non-social objects reflect genetic variation
- Authors: Ana Maria Portugal, Charlotte Viktorsson, Mark J. Taylor, Luke Mason, Kristiina Tammimies, Angelica Ronald, Terje Falck-Ytter
- Journal: Nature Human Behaviour
- Publication Date: November 27, 2023
- DOI: 10.1038/s41562-023-01764-w