Umberto Castiello and his team have shown that by the 14th week of gestation, unborn twins are already directing arm movements at each other, and by the 18th week these 'social' gestures have increased to 29 per cent of all observed movements. In contrast, the proportion of self-directed actions reduced over the same period.
Furthermore, the 'kinematics' of the twins' 'caressing' arm movements to each other's head and back were distinct from movements aimed at the uterine wall or at most parts of the their own bodies. That is, the social movements were longer-lasting and slower to decelerate than most other fetal movements, making them similar to the kind of movements fetuses learn to make towards their own eyes. This suggests that the fetuses recognise on some level that there is something special about their twin.
The researchers made their observations using four-dimensional (3-D plus changes over time) ultrasound scans of five women pregnant with twins. These were performed twice for twenty minutes - at the 14th and 18th weeks of gestation.
'The prenatal "social" interactions described in this paper epitomise the congenital propensity for sociality of primates in general and of humans in particular,' the researchers said, 'grounding for the first time such long-held intuition on quantitative empirical results.' Castiello and his colleagues added that further research of this kind could one day reveal the links between social behaviour patterns in the uterus and the later appearance of developmental disorders associated with social impairments.
Castiello, U., Becchio, C., Zoia, S., Nelini, C., Sartori, L., Blason, L., D'Ottavio, G., Bulgheroni, M., and Gallese, V. (2010). Wired to Be Social: The Ontogeny of Human Interaction. PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013199