The science behind sex determination in placental mammals (mammals, including humans, that have a placenta) is clear:

In placental mammals, the presence of a Y chromosome determines sex.

Normally, cells from females contain two X chromosomes, and cells from males contain an X and a Y chromosome.

Occasionally, individuals are born with sex chromosome aneuploidies [meaning, they are extra or missing], and the sex of these individuals is always determined by the absence or presence of a Y chromosome.

In everyday language:

  • a male has a Y chromosome;
  • a female does not have a Y chromosome.

When it comes to defining men and women, male and female, a distinction can be made between “sex” (a binary biological reality of being male or female, ordered toward reproduction, even if reproduction never takes place) and “gender” (psychological, behavioral, social, and cultural expressions of being male or female).

Gender activists are working hard these days to change federal laws and regulations (like Title IX), so that “sex” includes the social construct or psychological perception of “gender identity”—which guts the term “sex” of its meaning.

That has led other legislators to attempt to define “sex” in a biological way. But sometimes they do this in a way that is hasty and imprecise. They need the help of someone like analytic philosopher Jay Richards.

He says that a good and precise definition of sex will do three things:

  1. It will capture the central concept of biological sex—the orientation of male and female bodies for reproduction.
  2. It will refer to what happens under normal development—while accounting for disorders.
  3. It will accommodate the fact that organisms have and do different things at different stages of development.

Leave any of those three out, and the definition will suffer from imprecision and open the door to counterexamples and objections.

So what would be a good definition?

Here is what he proposes:

A human male is,


a member of the human species who,

under normal development,

    • produces relatively small, mobile gametes—sperm—at some point in his life cycle, and
    • has a reproductive and endocrine system oriented around the production of that gamete.

A human female is,


a member of the human species who,

under normal development,

    • produces relatively large, relatively immobile gametes—ova—at some point in her life cycle, and
    • has a reproductive and endocrine system oriented around the production of that gamete.

Note these two important caveats:

(1) Minimally. These definitions don’t say everything there is to say, but they say nothing false. In other words, everything they say is true, even if there is more truth to be told (e.g., about being created in the image of God for a particular mission).

(2) Under normal development. This acknowledges that abnormalities and disorders exist. We know, for example, that human beings are bipeds (under normal development, they have two legs). But we also know that if a chromosome or an event in utero disrupts the development of legs, or if a later accident causes the loss of legs, that we do not say that the person is a member of another species of “interspecies.”

So what do we do if a newborn lacks a secondary sex characteristic (like a penis) or has ambiguous genitalia or has a chromosomal anomaly?

Richards responds:

We would not, and should not, conclude that the child is not a human, or has no sex, or is some third sex. In most cases, we can with a bit more investigation determine that the child is male or female, and so would have the usual features of that sex except for a disorder that disrupted normal development.

Even if we could not determine the sex of an individual, we would treat this as an epistemic limit. We would not, or at least should not, treat such a person as a member of a third sex, or of no sex.

In short, Richards says:

  • Current efforts to redefine sex to include “gender identity” would dissolve sex as a stable legal category and create legal chaos.
  • In response, public institutions must shore up their defenses.
  • One key way to do that is by defining sex—including male and female—precisely in law.

You can read the whole thing here.

See also this piece by biologist Colin Wright in The Wall Street Journal.