A person emailed me recently to debate a definition of sex, ostensibly validated by Professor Emma Hilton at University of Manchester. Hilton's definition of sex is found on her Project Nettle website, and reads as follows:

Humans can be differentiated into two categories by their reproductive roles. Females make eggs and gestate live young. Males generate sperm to fertilise the female egg.

Those familiar with my work know I mostly agree with this definition. Sex must be reduced purely to the gamete contributed in a single act of reproduction, because any definition must apply to every species that engages in sexual reproduction. So far, so good, in my book, with the exception that Hilton already names gestation in what ought to be a general definition.

Hilton, however, takes her definition just a bit further:

In accordance with their respective roles, females and males have different reproductive anatomies (“biological sex”). No other reproductive mechanism exists in humans.

Now Hilton reduces the gamete contributed to an act of reproduction to the anatomy necessary to contribute it. She calls out humans specifically, and her staunch support of women's sports led to further expansion of her initial, (mostly) valid thought.

To be clear, I cannot find a reference in which Hilton herself states this hypothesis explicitly. When I heard it, it went as follows:

Sex is defined by the contribution of a specific gamete to an act of reproduction. The definition includes organisms with the potential to contribute a specific gamete; organisms that would contribute a specific gamete, but are broken; and organisms that historically could contribute a specific gamete to an act of reproduction.

The ontological identification of potential, broken, and historical with actual is beyond ludicrous. I devised several examples of how silly it is, each more absurd than the last (ask me someday about how this hypothesis would apply to a bank account). However, this hypothesis was advanced as a serious contender on which politics should be based, and I find it worth thinking about.

For now, suspend your ontological apprehension, give the hypothesis the benefit of the doubt, and let's analyze it from a scientific point of view.

Sex and genetics

From the American Heritage Dictionary, 5th Edition, "potential" means:

Capable of being but not yet in existence; latent or undeveloped.

If we confine our hypothesis to investigate humans, potential must include only organisms who have begun to develop reproductive parts. Hilton states as much on her Project Nettle website - that anatomy is primary in determining what she calls "biological sex."

But my hopeful debate opponent applied potential to the karyotype of a zygote at conception, when the entire existence of the zygote lies ahead of it. We know karyotype at conception does not define anatomy. In humans, every zygote has the potential to develop either male or female reproductive parts depending on the causal chain of human reproductive development.

That is, the mother ultimately determines the reproductive parts that develop in an embryo by how much testosterone she secretes into the embryo's environment. That testosterone may be induced by SRY protein transcribed by the SRY gene located on the Y chromosome, but that is not the only path possible.

And this is not theory, either - this is human development, found in any pre-med textbook.

Proponents of this definition claim an organism with a particular karyotype that does not develop specific reproductive parts is broken, not that potential is misused in cases that don't follow that path. There must be a close connection, then, between potential and broken.

We'll return to "potential" after investigating more of this definition.

Shoulding on the definition of sex

Application of broken demands a "should." An organism "should" develop according to a standard, regardless of what we observe.

In order for potential and broken to apply to a zygote at conception, we must possess an infallible predictive method to determine what an organism "should" do. At conception, however, a zygote is a single cell. A zygote is only its genetics, and the predictive method could only be karyotype - chromosome pair 23 characterized as XX or XY.

As stated above, biology observes karyotype is not infallible. Human reproductive development follows a long causal chain from karyotype to differentiation of reproductive parts, and the mother exerts a significant influence over that causal chain. Ironically (for my hopeful debate opponent), Hilton's work validates this knowledge of human development. She uses it as part of her argument that every male is biologically distinct from every female.

In order for potential to apply to karyotype at conception, we must include the mother in our predictive method. Of course, the mother is a product of her parents, and any predictive method must take into account her genotype, on and on to the origin of life. An added complication is that no methodology currently exists to find karyotype at conception. To do so would destroy the zygote, rendering the argument moot.

An infallible method to predict what a zygote "should" do is impossible, especially as the Y chromosome is to resistant to evolutionary pressure - to the point it appears to be evolving out of the human genome entirely.

But proponents of this definition appear to find karyotype suitable - "good enough." It works 95% of the time, they claim.

But is "most of the time" sufficient for a scientific definition? Or is this shortcut useful only to justify political agendas of those who would calcify human identity at conception?

Why broken is broken

The short answer is no, a definition that operates successfully 95% of the time is not sufficient for science.

Any hypothesis that cannot explain all observations of Nature is incomplete and must be revised. This is the Scientific Method at work - it has nothing to do with human development and everything to do with observing Nature honestly. If your hypothesis fails in one instance, it is a failure, full stop.

As a result, the only possible application of potential is when reproductive parts begin to develop in a potential human. Again, this reduces sex to anatomy, which is insufficient in itself as a definition, but is at least inherent in Hilton's work and her inconsistent definition.

Where does this leave broken? The description must imply an organism with apparent reproductive anatomy that doesn't appear to function as expected. In this case, that means reproductive anatomy that doesn't produce a gamete that could be delivered in an act of reproduction.

I emphasize "delivered" because it implies many humans born, Assigned Female at Birth based on obvious external anatomy, yet possessing 5-α Reductase Type 2 Deficiency - such as Caster Semenya, who is incapable of delivering gametes - are female. This is consistent with Hilton's own definition that sex is defined by anatomy.

Pity, that, if you're pushing for anatomy to define who competes in what event.

But how many children have you had?

Finally, we must address historical in this definition of sex. If we choose to exclude organisms that never contribute a gamete to an act of reproduction - even if we assume they "should" - and leave them to languish in categories potential or broken, a definition of historical is slippery.

Having contributed a gamete implies having reproduced at least once. "Should be capable" must infallibly produce yes or no to the question - independent of parent, dependent only on the human in question.

But there is no method to distinguish among potential, broken, and historical unless the organism attempts to reproduce at least once. Any organism that appears to be able to attempt reproduction might fit into potential, but the organism may also be broken, and - assuming the organism attempts to reproduce more than once, might be historical.

This is not a definition. This is a heuristic that demands the only purpose of a human be to reproduce.

Correction: the only purpose of a female human is to reproduce, as proving whose sperm is contributed in an act of reproduction is tricky without comprehensive genetic analysis.

To summarize, only a clear differentiation in development of reproductive parts could establish potential that could establish whether the parts are broken, which gives meaning to actual, which could develop into historical.

Same as it ever was...

The thing is…when we examine in depth the definition of sex as potential, broken, actual, and historical contribution of a gamete, we return exactly to where we are today. Today, at every birth that results in a certificate, a doctor examines an infant and takes a guess - based on the most obvious anatomy - of what might happen if the infant reaches sexual maturity.

The definition of sex we examined reduces to the judgment of a doctor, not any test based in science.

Again, this is at least consistent with Hilton's definition of sex as anatomy, but that definition leaves much to be desired.

If sex is a concept based on gametes, as Hilton claims (and I agree), it must stop there. It cannot be dependent on individual development of sex characteristics, because we observe infinite variation in expression of sex characteristics.

Hilton's canonical penis and vagina don't exist, and determining sex at conception is a pipe dream.

As described, despite the possibility of errors, the method of determining sex by visual inspection at birth was, has been, and is "good enough" - that is, good enough to observe that science has been discarded to satisfy what amounts to rationalization of political agendas.

If rationalization of politics seems like a disappointment after this much analysis of a hypothesis never intended to be scientifically valid, imagine the sleight-of-hand necessary to defend those politics in honest debate. Unfortunately, I believe this sleight-of-hand is purposeful.

Obfuscation is precisely how and why Executive Orders redefining terms from biology are written:

Where there is no science, there can only be legislation.