My dad claims that even as toddlers, my sister and I knew which toys to play with. For example, I would always pick up toy guns, Action Man, and other similar toys. She would always pick up Barbie. He also claims that this happened while we were very young, around 2 years old, before having been exposed to advertising and social biases. Is there any evidence that boys behave like stereotypical boys and girls behave like stereotypical girls absent social conditioning?
This has been studied in 2002 and 2009 at Texas A&M on monkeys and toddlers respectively, and the findings were that boys are genetically programmed to like trucks and girls to like dolls.
Findings from another study at Emory University, Atlanta in 2008 on monkeys also concluded the same.
The technique in the 2002 study on vervet monkeys was criticized by the Emory team (2008) who chose a different approach on rhesus monkeys.
The Texas team measured individual time spent with masculine and feminine toys shown to the subjects separately while the Emory team showed them masculine and feminine toys together and left the monkeys to choose between them
I have some original references to the mentioned studies and some media reports so far.
2009 study on humans From:
Alexander, G. M., Wilcox, T., & Woods, R.* (2009). Sex differences in infants’ visual interest in toys. Archives of Sexual Behavior, 38, 427-433.
In the research project funded by the National Science Foundation, psychology professor Gerianne Alexander used technology to track the eye movements of 30 infants ranging in age from 3 months to 8 months old. Alexander’s team set a doll and a truck in a puppet theater-styled box several feet in front of the babies, who were in car seats. The subjects couldn’t verbalize their preferences for the toys, but visual tracking monitors measured how long they fixated their attention on particular toys during two 10-second intervals. The girls favored the dolls, while the boys preferred the toy trucks.
“The existence of these innate preferences for object features coupled with well-documented social influences may explain why toy preferences are one of the earliest known manifestations of sex-linked social behavior,” Alexander explains in a paper titled “Sex Differences in Infants’ Visual Interest in Toys,” published in the journal "Archives of Sexual Behavior."
2002 study on vervet monkeys titled Sex differences in response to children's toys in nonhuman primates (Cercopithecus aethiops sabaeus) from Evolution and Human Behaviour Volume 23, Issue 6, Pages 467-479 (November 2002)
Sex differences in children's toy preferences are thought by many to arise from gender socialization. However, evidence from patients with endocrine disorders suggests that biological factors during early development (e.g., levels of androgens) are influential. In this study, we found that vervet monkeys (Cercopithecus aethiops sabaeus) show sex differences in toy preferences similar to those documented previously in children. The percent of contact time with toys typically preferred by boys (a car and a ball) was greater in male vervets (n=33) than in female vervets (n=30) (P<.05), whereas the percent of contact time with toys typically preferred by girls (a doll and a pot) was greater in female vervets than in male vervets (P<.01). In contrast, contact time with toys preferred equally by boys and girls (a picture book and a stuffed dog) was comparable in male and female vervets. The results suggest that sexually differentiated object preferences arose early in human evolution, prior to the emergence of a distinct hominid lineage. This implies that sexually dimorphic preferences for features (e.g., color, shape, movement) may have evolved from differential selection pressures based on the different behavioral roles of males and females, and that evolved object feature preferences may contribute to present day sexually dimorphic toy preferences in children.
In 2002, Gerianne M. Alexander of Texas A&M University and Melissa Hines of City University in London stunned the scientific world by showing that vervet monkeys showed the same sex-typical toy preferences as humans. In an incredibly ingenious study, published in Evolution and Human Behavior, Alexander and Hines gave two stereotypically masculine toys (a ball and a police car), two stereotypically feminine toys (a soft doll and a cooking pot), and two neutral toys (a picture book and a stuffed dog) to 44 male and 44 female vervet monkeys. They then assessed the monkeys’ preference for each toy by measuring how much time they spent with each. Their data demonstrated that male vervet monkeys showed significantly greater interest in the masculine toys, and the female vervet monkeys showed significantly greater interest in the feminine toys. The two sexes did not differ in their preference for the neutral toys.
The original 2002 studies were tried on rhesus monkeys in 2008 by a different team which also concluded that when given a choice between stereotypically male “wheeled toys” and stereotypically female “plush toys”, male rhesus monkeys show strong and significant preference for the masculine toys.
Horm Behav. 2008 August; 54(3): 359–364.
Janice M. Hassett, Erin R. Siebert, and Kim Wallen, of Emory University
Socialization processes, parents, or peers encouraging play with gender specific toys are thought to be the primary force shaping sex differences in toy preference. A contrast in view is that toy preferences reflect biologically determined preferences for specific activities facilitated by specific toys We compared the interactions of 34 rhesus monkeys, living within a 135 monkey troop, with human wheeled toys and plush toys. Male monkeys, like boys, showed consistent and strong preferences for wheeled toys, while female monkeys, like girls, showed greater variability in preferences. Thus, the magnitude of preference for wheeled over plush toys differed significantly between males and females
We offer the hypothesis that toy preferences reflect hormonally influenced behavioral and cognitive biases which are sculpted by social processes into the sex differences seen in monkeys and humans.
The one previous study of nonhuman primates’ interactions with human toys did not make subjects choose between masculine and feminine toys simultaneously available and thus could not directly measure preference. Instead they compared the relative proportion of interaction times with singly presented toys as a proxy for preference (Alexander and Hines, 2002). Comparisons between sexes found that the proportion of males’ toy interactions directed to masculine toys was greater than the proportion of females’ interactions directed to masculine toys.A similar, but opposite, difference was found for the proportion of interactions directed towards feminine toys, suggesting clear between-sex differences in preference for masculine and feminine toys similar to that seen in humans.
We investigated toy preferences in rhesus monkeys living in a 135 member long-term stable outdoor group by presenting the group with multiple trials of simultaneous access to different two toy combinations of multiple toys: one putatively masculine and one putatively feminine. We present here striking evidence of a sex difference in rhesus monkey preference for human gender-stereotyped toys paralleling that reported in humans, suggesting that gender differences in toy choice may reflect evolved sex differences in activity preferences not primarily resulting from socialization processes.
Scientists from Cambridge University found that boys and girls are different (sexual dimorphism) even from day one. They exposed infants one day old to a thing and a face, and measured differences in interest (looking time). Neither the test person nor the timer knew which sex each infant was.
The abstract from the 2000 study, "Sex differences in human neonatal social perception", reads:
Sexual dimorphism in sociability has been documented in humans. The present study aimed to ascertain whether the sexual dimorphism is a result of biological or sociocultural differences between the two sexes. 102 human neonates, who by definition have not yet been influenced by social and cultural factors, were tested to see if there was a difference in looking time at a face (social object) and a mobile (physical-mechanical object). Results showed that the male infants showed a stronger interest in the physical-mechanical mobile while the female infants showed a stronger interest in the face. The results of this research clearly demonstrate that sex differences are in part biological in origin.
The BBC produced a programme called "No More Boys and Girls", where they tried to do the opposite of most studies which attempt to measure the amount of gendered behaviour in infants or monkeys.
The programme included the opinions of experts in the field, who suggested that while male and female brains are different from birth the differences are relatively minor. Furthermore, many masculine and feminine attributes attributed to biological sex are in fact either due to environmental factors or simply don't exist.
Examples included "spacial awareness", which experts and experimental evidence suggested was more due to girls not getting as much practice during play than any genetic factors. Later the physical strength of boys and girls aged 7 was shown to be equal (accounting for size), only diverging with the onset of puberty, despite almost everyone assuming it exists from birth.
Another interesting experiment had child care workers being told that infants were the opposite gender to their biological sex. Naturally they guided play towards toys they associated with the gender they had been told, and the children played with them happily with no visible signs of being instinctively drawn to the toys "matching" their true gender.
Overall the programme and experts interviewed concluded that genetic differences, while having some influence, were generally very minor and the vast majority of human behaviour is influenced by social factors.
This idea came up again when James Damore wrote his now infamous memo. Wired interviewed the authors of the science he cited as evidence to back up his claims, and they largely refuted his conclusions on the same grounds as the BBC found - that nurture and social influences are vastly greater than very minor genetic ones.
In conclusion, yes there is some evidence but it only demonstrates a very minor influence, and there is a great deal of evidence also showing that right from birth social influences are primary influence.
In addition to the answers already given, there is also the tragic case of David Reimer, who was born a boy in 1965 but suffered a botched circumcision and was subsequently given gender reassignment surgery and raised as a girl. The psychologist John Money claimed that Reimer was successfully being raised as a girl and was exhibiting stereotypical female behavior such as being happy to wear girls clothes and preferring to play with dolls rather than toy trucks. He presented this as evidence to support the theory that behaviour differences between girls and boys are the result of socialisation rather than being innate.
However in 1997 Reimer went public with his own story. He denied that he had ever enjoyed playing with dolls, stated that he had never felt like a girl, and that Money had ignored or suppressed all evidence that was counter to his socialisation theory.
While this is only one case, it does strongly suggest that the behaviour differences between boys and girls are innate rather than socialised.
The reference I gave above is to a Wikipedia page on the case, but the page has plenty of references to primary literature, and I think its better to point people there rather than try to duplicate it here.