Evolutionary scientist Kelly McLain and colleagues from Georgia State University argue in the journal Evolution and Human Behaviour that if fathers question whether their children look suspiciously different from them, mothers and offspring could find themselves abandoned.
Worse still, the baby and mother could be vulnerable to violence from a man for whom an unfamiliar-looking baby proves he has been cuckolded. After all, it makes poor evolutionary sense for a man to protect another male's genes.
Recent estimates say that as many as 30 per cent of putative fathers are not the genetic parent, so mothers needed a way of ensuring they are not cast out. The solution for most is to persuade fathers that babies look like them - even when they do not.
Dr McLain and colleagues demonstrated this by showing photographs of newborn babies to independent judges and mothers, asking if the babies most resembled mum or dad. The judges matched photos of newborn babies significantly more often to photos of mothers than fathers.
This occurred at a much higher rate than any previous estimates of cuckolding could account for. In other words, to independent observers, most babies resemble their mothers rather than their fathers.
Yet mothers were inclined to say their babies looked like the father rather than themselves. This happened most often if the father was present when the researchers asked the question.
The bias does not reflect resemblance, so scientists speculate it may be an evolved response to assure fathers of their paternity.
But why should nature have ensured that babies tend to resemble mothers more than fathers? "Natural selection may have favoured concealing paternity in babies' appearance - concealment reduces the risk that domestic fathers will react aggressively towards children who do not look like them, which may be the case with some degree of marital infidelity," said Dr McLain.
Concealment of babies' resemblance to fathers is, then, favourable to mothers and infants, but only assuming a fairly high rate of cuckolding by women.