I don't have the original study I read (back in a cognitive development class in college), but I believe this one[0] should suffice. The original was a study that monitored groups of kids playing with toys and found the kids didn't develop a gendered preference until parents began interfering with toy selection. If the parents didn't interfere, they wouldn't develop a gendered preference until school age where they began interacting with other children.

[0]: http://onlinelibrary.wiley.com/doi/10.1111/j.1540-4560.1990....