What this suggests is that not only are male pronouns used as generics, as would be expected, but also that writers tend to write about males more often than females. In fact in Graham's study males were referred to seven times as often as females (Graham 1973 in Thorne et al 1983). It is not far-fetched at all to suppose that part of the reason for this phenomenon is the prominence of the male pronoun. An additional factor is the widespread use of "man" to derive sexist job titles from occupations (e.g. "Salesman" from "Sales", "Businessman" from "business" etc.). Once again this is a very obvious objection but the repercussions are larger than one would think. Silveira (1980 in Thorne et al 1983) found in a series of studies that while women tended to interpret gender-biased "generic" pronouns as generic men tended to interpret them as meaning "male" and that this included terms that simply contained the word "man". Silveira also found that both men and women tend to visualise male figures when they read the masculine "generic" or hear it being used. With this in mind it is easy to see why writers in general tend towards describing males even when the situation is wholly ambiguous (as in the example of animals in children's school books cited above).
Thorne et al (1983) points out another kind of linguistic sexism that more even clearly demonstrates the social disempowerment of women in the English language. This is the tendency to refer to women in terms of their relations to men. While the argument still rages as to whether (and how) women should take their husband's names when they marry Thorne et al points out that there are many other instances of this tendency. For example referring to a woman as "Harold's widow". It would be extremely unlikely for anyone to refer to a man as "Jane's widower" even if the term "widower" was still used in our society. In fact Wardaugh (1992) believes that the fact that terms like "widower" and "spinster" have died out while terms like "widow" remain popular is another example of this trend; the tendency to use language to enforce power relations.