I was privileged to attend Maya Forstater’s employment tribunal last month which was incredibly interesting and also gave me some helpful pointers for my own legal case.

Sadly members of the public are not allowed to stand up in employment tribunals and say “Please sir, please Mr Judge, I know the answer” but I was sorely tempted to give it a go. Maya had persuaded the barrister for her ex-employer to stop saying that sex is “assigned at birth” but rather is observed from a baby’s genitals (and, rarely, if necessary, through DNA testing).

The barrister went on to ask how Maya could tell that someone is male or female without having been present at that person’s birth. Happily the rest of us live in the real world where it is pretty easy to work out whether someone is male or female: and no, it doesn’t depend on whether they have highlighted cheekbones and manicured fingernails.

So, how do we tell whether someone is male or female? Well, we are very good at it and — like a lot of human cognitive skills — we base it on a number of cues (pieces of information). In fact, we’re very good at it from the time we are tiny babies.

One of the best ways to tell a male body from a female body is gait — how you walk. You don’t need a whole body in front of you, or even the outline of a body, to tell male from female bodies. A nice little point light display will do the trick. Adults and babies aged 4 months or older can tell male from female in this type of video (first image shows a still, second is a video like the ones used in this type of experiment).

A point light display showing female (left) and male (right) adults with a dot of light on their limbs.

(Click through on the video if you want to confirm you can tell male walkers from female walkers. Sadly I cannot find a video for the other sex!).

Next up, we can tell the difference between male and female faces. Just as with gait, both infants and adults can do this.

For babies, we usually work out what they can distinguish by finding out what they prefer to look at and what they prefer to look at when we’ve bored them into submission which is technically known as “habituation”. If a baby sees something that looks the same to them, after a while, they will stop looking (I imagine them saying “OK, stop now, I’ve seen that, show me something NEW and EXCITING). This means if we switch in a new picture or sound and they now pay attention, we usually decide they can tell the difference between the old (BORED NOW MUMMY) picture or sound and the new (WOW THAT’S EXCITING) picture or sound. But babies also like to look at some things more than others. Babies who spend more time with Mum or a female caregiver can tell the difference between male and female, and prefer female. But babies who spend more time with Dad or a male caregiver can also tell the difference, but prefer male.

For adults, there seems to be a “prototypical male” and “prototypical female” face and we seem to use a variety of features to distinguish: feature size and shape but also the distance and configuration of facial features.

As an aside, most of the research I’m quoting here says “gender” differences instead of “sex” differences. I wrote before how the use of “sex” in research articles has gone down over time as the use of “gender” has increased, but for example in the article I’m quoting here — O’Toole et al. — the details they give e.g. size, shape, and configuration of facial features — are determined by testosterone both before birth and during puberty. Note that article says “faces are sexually dimorphic” but still uses the word “gender”.

Different dimensions that can be measured on faces to distinguish male from female

And stop press! (Ok it’s fairly old but it’s fun…) Neural networks can tell male and female faces apart (in fact, they do slightly better than humans, but make some of the same kinds of mistakes).

But wait! Infant monkeys can also tell male and female faces (monkey faces — but not human faces) apart. I’m a bit torn here whether to show you a cute photo of an infant macaque monkey or tell you how mean they can be (I’ve had many a lunch stolen by a macaque and my friend was bitten in an area you DON’T want to get bitten). OK… let’s go with cute.

I’ll move on now to voices, though I could go on and on, this is a particularly helpful method for situations where you can’t see the whole of a person’s body and/or it’s dark and/or they are on the phone.

Again, babies can tell male from female voices. They may be able to tell the difference at birth (and they seem to prefer women’s voices — very likely because they have heard mainly a female voice before birth) but they are much better at categorisation (telling the difference between the broad categories of male and female rather than between individual speakers) by the time they are six months old. There is an interesting study suggesting they may even be able to tell male from female voices before they are born.

When adults are listening to male and female voices, we’d be tempted to think that the main difference was the pitch (fundamental frequency, which is more or less the same thing only in tech terms). Obviously this does differ but there are other sound-based differences we can hear and use: breathiness is one of these.

In fact, when asking adults to say how similar they thought two speech recordings were, the main factor they used to judge this was the speaker’s sex — even though they could hear lots of other factors such as the effort a speaker puts in, how nasal their voice is, and how hoarse they were.

So: we can tell an adult man from an adult woman based on a lot of things. Now, some of these things can be changed in an adult who has medical treatment. Most (but not all) women who identify as men and have testosterone treatment go on to have lower voices.

This doesn’t seem to be reversible: if you are a man who has had the usual surge of testosterone at puberty, only surgery can change your face or your voice. Though speech therapy can make a bit of a difference in your voice, it can also sound a bit… er… odd.

Video of speech therapy services for trans people

The video (which is from a speech therapy service showing how they work with trans people, so I assume the client is happy with it being public) shows how it’s possible to use a higher pitch voice but that this could actually be bad for the vocal cords, and to have speech therapy to change the voice. They’ve chosen not to simply raise their voice to a higher pitch, but to use a different way of speaking that they personally prefer. As I said above, the difference between male and female voices is not simply pitch, but we use pitch a lot to distinguish them, so it’s unlikely that a person wanting a more “feminine” voice who doesn’t alter pitch will be mistaken for a woman. You might want to have a look at the segments of walking, too, to see what a biological man walks like, except of course… you already knew — you’ve known since you were a baby.