One experiment we conducted here in Cambridge was at the local maternity hospital. Essentially we wanted to find out whether sex differences that you observe later in life could be traced back to birth, to see if such differences are present at birth. In this experiment we looked at just over one hundred newborn babies, 24 hours old, which was the youngest we could see them, and we presented each baby with a human face to look at, and then a mechanical mobile suspended above the crib. Each baby got to see both objects.
Obviously these objects are different in interesting ways, because the human face is alive, and it can express emotion, it's a natural object. The mechanical mobile is man-made, it's not alive, and obviously it doesn't have emotions. We tried to make the two objects equivalent in some important ways. One is that they were both the same size; another was that they were a similar colour, in order to try and control features that might be grabbing the child's attention. But effectively what we did was film how long each baby looked at each of these two objects.
We asked the mothers not to tell us the sex of their babies, so that we could remain blind to whether this was a boy or a girl. And for the most part that was possible. Sometimes it was possible to guess that this was a boy or a girl, because there would be cards around the bed saying, "Congratulations, it's a boy." That potentially could have undermined the experiment, although we then gave the videotapes to a panel of judges to simply measure how long the baby looked at the face or the mobile. By the time the judges were looking at these videotapes they didn't have any of these potential clues to the sex of the baby, because all you could see was the eyes of the baby.
The results of the experiment were that we found more boys than girls looked longer at the mechanical mobile. And more girls than boys looked longer at the human face. Given that it was a sex difference that emerged at birth, it means that you can't attribute the difference to experience or culture. Twenty-four hours old. Now you might say, well, they're not exactly new-born, it would have been better to get them at 24 minutes old — or even younger. But obviously we had to respect the wishes of the parents and the doctors to let the baby relax after the trauma of being born. And let the parents get to know their baby. So strictly speaking, it might have been one day of social experience. But nonetheless, this difference is emerging so early that suggests it's at least partly biological.
