When toy shopping for a child, we usually know that a boy might prefer a toy truck, while a girl would enjoy playing with a doll—but at what age does it start?

Academics at City University London and UCL set out to study this phenomenon and found that children as young as 9 months prefer to play with toys specific to their gender.

The paper, published in the journal of Infant and Child Development, shows that in a familiar nursery environment significant sex differences were observed at an earlier age than gendered identity is usually demonstrated.

“Sex differences in play and toy choice are of interest in relation to childcare, educational practice and developmental theory. Historically there has been uncertainty about the origins of boys' and girls' preferences for play with toys typed to their own sex and the developmental processes that underlie this behavior,” Brenda Todd, Ph.D., a senior lecturer in psychology at City University, said in a statement. “As a result we set out to find out whether a preference occurs and at what age it develops.”

The research postulates the possibility that boys and girls follow different developmental trajectories when it comes to choosing gender-typed toys and that there’s both biological and developmental-environmental components to the sex differences seen in object preferences, according to the study.

To investigate gender preferences in terms of toys, the researchers observed 101 boys and girls in three age groups—1) 9-17 months, 2) 18-23 months, 3) 24-32 months in U.K. nurseries sans the presence of a parent. The toys used in the study were a doll, a pink teddy bear and a cooking pot for girls, while for boys a car, a blue teddy, a digger and a ball.

Stereotypical toy preferences were found for both boys and girls in each age group, demonstrating that sex differences in toy preference appear early in development. However, both boys and girls showed a trend for an increasing preference with age for toys stereotyped for boys, according to the study.

“Biological differences give boys an aptitude for mental rotation and more interest and ability in spatial processing, while girls are more interested in looking at faces and better at fine motor skills and manipulating objects. When we studied toy preference in a familiar nursery setting with parents absent, the differences we saw were consistent with these aptitudes,” Todd added. “Although there was variability between individual children, we found that, in general, boys played with male-typed toys more than female-typed toys and girls played with female-typed toys more than male-typed toys.”