Babies are born with an innate ability to tell the faces of different primates apart.
Research has shown that at six months an infant is as adept at differentiating between simians as they are humans.
However, by nine months, it seems this ability is lost. They become as inept as adults at picking a familiar monkey from the crowd.
Olivier Pascalis, a lecturer at Sheffield University, and colleagues at University College London and the University of Minnesota in the US, believe this is because face processing in the brain is tuned by the faces people see the most in the first year of their lives.
Their results are published in the latest issue of the journal Science.
The scientists familiarised 15 six-month-old infants with a picture of a face. They then showed them the same image paired with a new one.
The scientists noted the babies' eyes tended to fixate longer on the unfamiliar image, whether it was a human or a monkey.
The experiment was repeated with nine-month-old infants and the researchers found that this time the effect only occurred in relation to human faces. Monkeys had become practically indistinguishable. The same result emerged when adults took the test.
Dr Pascalis said this suggested that at six months, an infant is still comparing any face it sees - human or monkey - with a basic template that is "hard wired" into its brain.
But as it develops, its face processing system becomes more sophisticated. It hones in on the perceptual differences between the faces the baby sees around it and loses the ability to distinguish others.
The same pattern is echoed by the declining ability to learn a new language with age.