In her second chapter, Stock sketches three somewhat plausible theories about what sex is, that is, what it is to be male or female.

The gamete account (advocated by Byrne) is meant to work across all sexually-reproducing species, and says that a male produces many small and mobile gametes, whereas a female produces (relatively) fewer, bigger, and less mobile gametes. Or rather, to be of one sex or the other the creature is or normally would have been on a developmental pathway to such productions; actual fertility is not required. Interestingly, this allows that a creature may, as happens with a few species, change sexes. Here, chromosomes are not relevant.

Second, there’s the chromosome theory of sexes, which tries to define what it is to be male or female only for human beings. This says that a human is male if it has a Y chromosome, otherwise it is female.

Stock notes (pp. 48-49) that both accounts seem to give what probably most of us would consider a wrong answer in the case of a person with XY chromosomes but who suffers from Androgen Insensitivity Syndrome, resulting in their having (at least somewhat developed) testicles, but externally, a typically female body. A few other Disorders of Sexual Development result in seeming counterexamples to each account too. For her part, Stock seems to want to just “bite the bullet” and accept each unintuitive classification as a consequence of what is overall a plausible theory. To this reader, such a stance is unsatisfying. Don’t we want to sort all the items in the collection correctly?

The third, “cluster” account is not so simple. Basically, the idea is that no underlying essences underlie being male or female. Rather, associated with each concept is a cluster of bodily characteristics – you know them. A person counts as one or the other if they have “enough” of one set or the other, and no one characteristic is required. Stock notes that this will give more plausible sortings when it comes to some developmental disorders (p. 53). At the same time, there will be some borderline cases where we’re not sure which category a person should go into.

We can’t get away from the fact that an overwhelming majority of humans can be easily sorted into one class or the other. That humans come (at least for the most part) in these two kinds is obvious, and is something that invading aliens would immediately observe about our species. They would not need to ask (at least most of) us which one we are! It’s not really much harder than the case of peacocks vs. peahens. It’s just that with us, things are the reverse; the females are the fancier ones.

Obvious though this seems, some recent thinkers have waged war on the facts, which Stock discusses later in this chapter . . .