causal chains from biology to behavior in sex and gender issues is very far from complete. There are studies concerning women who have a condition called congenital adrenal hyperplasia, which leads to the overproduction of the masculine sex hormone, androgen. These women usually have ordinary female appearances (though nearly all girls with congenital adrenal hyperplasia (CAH) have corrective surgery performed on their genitals). However, despite taking hormone-balancing medication given to them at birth, these females are statistically more likely to be interested in activities traditionally linked to males than female activities. Psychology professor and CAH researcher Dr. Sheri Berenbaum attributes these differences to an exposure of higher levels of male sex hormones in utero. Non-human animals According to biologist Michael J. Ryan, gender identity is a concept exclusively applied to humans. Also, in a letter Ellen Ketterson writes, "[w]hen asked, my colleagues in the Department of Gender Studies agreed that the term gender could be properly applied only to humans, because it involves one's self-concept as man or woman. Sex is a biological concept; gender is a human social and cultural concept." However, notes that the question of whether behavioural similarities across species can be associated with gender identity or not is "an issue of no easy resolution", and suggests that mental states, such as gender identity, are more accessible in humans than other species due to their capacity for language. Polani suggests that the potential number of species with members possessing a gender identity must be limited due to the requirement for self-consciousness. Jacques Balthazart suggests that "there is no animal model for studying sexual identity. It is impossible to ask an animal, whatever its species, to what sex it belongs." He notes that "this would imply that the animal is aware of its own body and sex, which is far from proved", despite recent research demonstrating sophisticated cognitive skills among non-human primates and other species. has also stated that whether or not non-human animals consider themselves to be feminine or masculine is a "difficult, if not impossible, question to answer", as this would require "judgements about what constitutes femininity or masculinity in any given species". Nonetheless, she asserts that "non-human animals do experience femininity and masculinity to the extent that any given species' behaviour is gender segregated." Despite this, Poiani and Dixson emphasise the applicability of the concept of gender role to non-human animals such as rodents throughout