Children have strong preferences for sex-typed toys; boys prefer trucks, whereas girls prefer dolls. These preferences appear to be driven by complex interactions of hormones and the socio-cultural environment. The relative contribution of each of these factors in children is impossible to isolate given ethical limitations. Non-human primate species afford the opportunity to examine preferences in the absence of societal values and influences that children experience. In two previous studies with non-human primates, one with vervet monkeys and one with rhesus monkeys, monkeys showed sex-typed object preferences that paralleled those of children. However, several uncontrolled variables could have influenced these preferences. Our study considered object characteristics and we controlled for possible color preferences. We also tested monkeys individually to eliminate the effects of social facilitation and dominance rank. In experiment 1, monkeys were given a choice between similar objects of different colors (Phase A) and moving vs. non-moving objects (Phase B). In experiment 2, monkeys were given a choice between dolls and trucks (Phase A) and subsequent phases looked at the influence of moving wheels (Phase B) and hardness (Phase C). Contrary to previous findings, monkeys did not show sex-typed object preferences. Instead, the monkeys preferred blue objects, hard PVC objects such as trucks and hard dolls, and dolls with wheels. The influence of previous reward based cognitive testing, familiarity of substrate materials, and rearing condition are considered as possible explanations for these findings
