Sex is a physiological concept -- it is biology. Physical and tangible. You can touch sex, and thanks to nerve bundles that have remarkably high induction rates and a brain that is incredibly cool, doing so often feels good once you get rid of all the social stuff.

In our culture, we tend to group people into two broad categories. Male and Female. We are taught, from the youngest age, that they are different, and then we go to school, and we learn that yes, indeed, they are different. And I don't mean the social stuff, again -- I mean the biology -- the stuff that is two people lying on a table.

We classify many creatures on different rules. Some we classify according to chromosomes, others according to gametes (who has the ovaries or the testes). Since a lot of species out there have all kinds of variations in that, it can get pretty complicated -- but almost all of them are only known to be that way because of work done in the last 25 years or so.

The most common way people are classified is by what's called the Primary Sex Characteristics. We usually call them genitals. This is what doctors use when we are born, and its truly amazing they get it right as often as they do. If you've ever looked at a newborn infant boy and girl's genitals, they can sometimes be very hard to tell apart.

That's been the custom in our culture and its predecessors for thousands of years. In the overwhelming majority of situations, it's correct, easy, simple, and obvious.

There is also the reproductive system. Testes and ovaries, for example, as noted above. There is the absence or presence of the Y chromosome or the X chromosome. There is the SRy gene (that carries pretty much all the "male" stuff in it). There are Secondary sex characteristics: breasts, hips, mounds, facial hair. There is skeletal structure. There is brain structure. And there are hormones.

However, genitals are only part of what we actually know goes into defining someone's sex. Indeed, as Eric Vilain pointed out nearly a decade ago, there are a great many ways of determining sex. All of them vary and all of them serve a particular purpose or goal.
The five most important ones, for the purpose of this column, will be these:

• genitals
• secondary sex characteristics
• gonads
• hormones
• chromosomes

It should be noted that sex determination is an old field, and that most of what we do know about it comes from studying people who are not typical -- who differ in some way from the usual, commonplace person for whom sex really can be determined by checking genitals.

This is normally, and historically, intersex people; but in the last 20 years LGBT folks have been looked at as well, as they give additional insight into wider questions, and as people used us to verify or cross check or prove and disprove certain ideas.

There are two IS conditions in particular that are important: Klinefelter's and Turner's. People with these conditions are XXY and generally look male, or X, and look female. So one has more chromosomes than usual, and one has fewer.

It was the study of the mechanisms that caused them that led to the discovery of a gene, in 1992, called SRy. SRy is, ultimately, what determines if someone will be male in terms of physiological development. And, although to my knowledge no one has been shown without it thus far, SRy can be carried on either the X or the Y chromosome. So it is possible for someone to be XX and male, and YY and female (although a YY person would, generally speaking, have an extremely short lifespan, due to additional factors found in the X chromosome that are essential to everyone.).

At least, as far as we know so far. Every year we learn more that changes how we see things.

And there's the rub. So long as we seek out a singular way of identifying between the two sexes, we will always tend to have some variation and some failure to include everyone.

As a result, many folks in the general fields surrounding Sex Determination have started to use a methodology that relies on the five factors I described above. It's not super widespread yet, but slowly gaining a wider acceptance. The short version is that it takes a majority of those five things to establish a person as physiologically male or female, and the particulars of that majority are what counts.

So if a person has the genitals of a male, the secondary sex characteristics of a female, the gonads of a male, the hormones of a female, and the chromosomes of a male, they would be male physiologically.

If they were to be missing, say, the gonads, then they would be female, since the system tends to learn towards the female, which is a problem it has, as it reinforces the idea entrenched still, which is sex dimorphism (the development of two sexes).

When it comes to trans people, this can be a little problematic. Fortunately by adding two other factors into it, we get a clearer picture that accounts for greater variance:
Sex Identity and Gender Identity.

One of the things people generally are not aware of yet, is that Gender Identity and Sex Identity are both present in every person, and that they are both inherently physiological. We know, for example, that the source is an area of the brain located central bottom. Indeed, it very close to the area that has been shown to have a high correlation of sexual orientation, as well.

They aren't the same areas, mind you, but they are very close to each other -- close enough that one can affect the other.

Most of the readers here will have heard of Gender Identity. They may even have a good idea about what it is, but the surprising thing is that they don't know about Sex Identity or what is.

Gender Identity I will talk about more in the next article, on Gender. Sex Identity, however, is important. Sex identity is how we see ourselves in terms of male or female. It is our personal understanding of that concept, void of any external influence. It is not something taught to us -- people have had accidents that strip their bodies of any way to sex them, and they still know. It is not founded in the flesh we can see. Sex Identity will be found, like Gender Identity, in later columns.

Now, a lot of people right now are saying "But that's gender identity". And, in truth, that's what most people think it is, except in the rarified world of academics and scientific study.

Nor is Sex identity anything new -- it's been around for about 50 years, and was developed at the same time as Gender Identity, but it's only been the shift in the last 20 years as people turned to look at what gender really is that we've come to see the value of using it and only a few locations (the University of Minnesota, for example) using it.

In a more direct form, linked to a recent column here, Sex Identity is what transsexuals experience as "where's my penis?" or "where's my vagina". In my case, it's rather disconcerting to know I'm a female, find out females have vaginas, and then wonder "hey - where the hell's mine?"

Gender and Sex are not the same. Gender is about others. Always. Sex is not.

So if you add Gender Identity and Sex Identity into the mix, you begin to see a greater clarity, and you account for the bias problem of it leaning towards female.

Now you have 7 factors, the majority of which determine one's sex, all of them tied to one's biology. Many of them are not going to be fully visible until the individual is into puberty, but we can make a pretty good guess otherwise.

Let's take a female to male transsexual as an example. You have a person with female genitals (assuming no surgery), the hormones of a male (thanks to testosterone), the secondary sex characteristics of a male (assuming top surgery), the gonads of a female, the chromosomes of a female, the gender identity of a male, and the sex identity of a male.

The balance is very much a male, in terms of biology.

For most people, the question seems rather absurd -- why does it matter. But for trans folk and intersex folk, it makes a huge difference, because we get caught in the middle ground and suffer from oppression, maltreatment, and other terrible things as a result.

Now, if you note carefully, there was no mention of man or woman in this discussion on sex. Nor masculine and feminine. This is because man and woman, feminine and masculine are parts of gender, not sex. Men do not have penises all the time, women do not always have a vagina.

This particular concept of sex determination, like my earlier taxonomy of Trans, creates a system whereby there are a much wider variety of possibilities, all still fitting into how we define tings - even if we are still stuck, at present, with the limitations of only two words to describe them all.

By carrying this idea with you when dealing in the LGBT and its associated communities, you begin to see how, just as with my last article, all of us are different, and yet all of us are the same.

Even when sex is between the legs.