> If the "simplistic" male/female binary explains >99% of observations, there does not seem to be a strong case for describing biological sex as a "spectrum"...

I'd suggest that it all comes down to how we're making those observations and what we're then doing with the classifications we've derived.

Why are we trying to describe the idea of biological sex? If it's for the sake of a scientific classification, then simplistic is a horrific idea. Biology by definition (in a modern world) is dealing with the small scale deviances and the nitty gritty. We're 99% rat, but the differences are somewhat remarkable.

We've been observing the wrong thing for thousands of years. Your assertion is almost correct; 99% of the time the _outcome_ of observation might be male or female, but it doesn't actually _explain_ anything. All it does is answer the question "does it have a penis"[1].

Which in 2015 has far less medical relevance than it did in centuries gone. "Well Mr. Smith, she is of the weaker species, this little touch of the vapours is to be expected, of course." is a diagnosis heard less and less. We're now increasingly diagnosing and categorizing conditions a genetic level, never-mind just what your second sex chromosome is. [2] From a technical point of view, it's already far more complicated than male/female. Simplifying science to that degree does it a disservice.

The big problem with binary though is the way it permeates through culture, language society and even thought.

Essentially, at birth, we clumsily label and categorise people based on their sex organs and worse, refuse to officially acknowledge the grey area. It's like asking someone with severe allergies if they want the shell fish or the peanuts. "Oh I'm sure they would both kill you, Sir[3], but you will be eating one for your entrée"

Put bluntly, I don't effing understand why everyone is so interested in what I have down my pants? You'd think it's entirely irrelevant except for some very specific circumstances, but instead we all have to publicly broadcast this binary allegiance all day every day. Everything. Pretty much every trivial form online makes me choose between "Mr", "Mrs", "Miss" or "Dr"[4] and usually as a mandatory field and yet you really don't need to know if I have a labia or not to be able to print my name and address and post me a parcel. It doesn't need to announce this on my bus pass or in nearly every english pronoun.

Through childhood I repeatedly went through conversations along the lines of "Which football team do you support?" I don't. I don't watch football. "That's fine, but which team do you support?"[5]. Not having an allegiance, even a lapsed one, is not an option.

By simplistically describing homosapien sex as binary it means anyone who doesn't conform is left out. It's not a legitimate option. Not viable, abnormal, mistakes, a freak of nature if you will. We're essentially saying that these people are inhuman and unnatural, which is obviously untrue.

The UK has allowed people to change their legal gender for over 10 years now[6], which legitimises the idea that external observation and internal state might not be linked. Simplifying that disparity to a binary state though, does a great disservice to the human condition. It seems odd that it's now legitimate not to identify as M||F but you've still got to identify as M||F.

"No", "Other" or "I don't know" should be a completely legitimate answer in response to sex or gender. It needs it's own category, it needs words to identify it and language to be able to describe it otherwise it's not a real thing, it's just anomalous.

And anyway, I'm disappointed; this is hacker news. Define it as either binary value or a boolean, but we should all be aware that as well as TRUE or FALSE, 0 or 1, variables can legitimately be undefined and it's something you have to be capable of handling.

apologies for the rambling answer.

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[1] Admittedly a bit glib, but ignoring the last 120 years of science this is probably the most distinct observation we've been able to make of a live subject. Talking points might include testis and eunuchs, the ability to give birth and the difference between sex, gender and gender stereotyping... oh, and sea horses.

[2] http://www2.le.ac.uk/departments/genetics/vgec/highereducati... vs http://en.wikipedia.org/wiki/XY_sex-determination_system

[3] Penis

[4] which until rather too recently has basically meant "penis", "already owned by a penis", "has no rights, available to be owned (warranty void if introduced to thinking)" or "even more respected penis".

[5] Feel free to make your own analogies; I recommend some of the misunderstandings between religion and atheism or the futility of voting for a political party you don't like as starting points.

[6] http://en.wikipedia.org/wiki/Gender_Recognition_Act_2004