A geeky thing that bugs me. Previously posted as a comment here.

They like saying "sex is a spectrum".

I suppose what they're trying to say is that sex is kind of like colours - it's a bit squishy and there's no clear dividing line between colours.

The potential truth in that is that a very rare individual might end up with their sex characteristics not totally normally developed. So someone could be somewhere "inbetween", with some characteristics of each sex. The "intersex" model of DSDs. It's a bad model, but let's roll with it for the metaphor.

Look at the colour diagram for this post, borrowed from Wikipedia.

That's a standard chart used to represent all the colours that humans can see. It's based on the human visual system - it's how we we see the world. We have 3 colour receptors, and you can map our colour response to a 2D diagram like that. Any colour can be expressed as a co-ordinate on there ("chroma"), plus a brightness ("luma").

The light spectrum you see naturally in a rainbow is a bunch of different light frequencies from 380nm (blue) to 700nm (red). Those are plotted along the curve at the top edge of the diagram. Pure single-frequency colours.

Our visual system can't tell the difference between a pure colour and a mixture, because we only have those three colour receptors (red, green, blue).

The pure colour yellow tickles our red and green receptors. We have no yellow receptor. So we can fake pure yellow by a mixture of red and green. And it looks like pure yellow to us. Either pure yellow or the mixture activates our red and green receptors.

The diagram shows that working by yellow being the midpoint between red and green. As you fade from red light to green light on the right edge, you move on a straight line through the colours, red, orange, yellow, yellowy-green, green. And then you can fade from green to blue, along that left edge.

And those red->green, green->blue fades simulate the spectrum using our eyes. Using only 3 colours. Not the infinite number in the real spectrum.

And that's how TVs and other display technologies work. Although they don't use totally pure colours, so they're limited to a smaller triangle, delimited by less-pure "red", "green" and "blue" primaries.


Moving along the top two edges of the triangle simulates a spectrum. Just not quite as vivid - TVs can't produce as vivid colours as nature. (Modern TVs can do more).

But colours in nature aren't just the pure colours in a rainbow. They can be mixtures too. Any colour not on the spectrum line must be being produced by a mixture.

Now look at either diagram. The pure spectrum (or the TV simulation of it) goes along the top. But what's that line along the bottom?

It represents the way our eyes respond to a mixture of red and blue. And those colours aren't in the spectrum. They're mixtures. There's no magenta in the rainbow. You can't get that "red + blue" response from our eyes with a single light frequency. You have to mix red and blue.

And that's where we get to the point.

IF sex was this "continuous line from red to blue". It would be that red->magenta->blue fade line. It would NOT be the spectrum along the top.

There are no intermediate frequencies. It's just different mixtures of red and blue.

And then, of course, on top of that, the "intersex" people who are "inbetween" are such a vanishingly small minority that a population plot would look like this:


The dividing line is so sharp you wouldn't even spot any boundary fuzziness at that scale.

Lecture over. I welcome questions.

  1. That graphic is mesmerizing on its own.

  2. I didn't know any of that colour theory except for the basic colour receptors in our eyes being RGB. I love learning new things and you are a good teacher!

  3. This was a really apt analogy to the sex issue, from my perspective. Thank you!

You're welcome. I just like talking sometimes :)

Have another bit of science.

The line across the middle of the diagram is another natural set of colours - the colours that hot objects glow, with corresponding temperature markings.

As you get into the 1000s of degrees, you're talking about the surface of a star.

The surface temperature of our sun is (currently) 5800K.

And that is what ultimately determines what we regard as "white". The "midpoint" of the colour space.

After atmospheric scattering, the light from the surface of the sun is just a teeny bit bluer than it is in space - equivalent to 6500K.

And then that colour corresponding to 6500K is our standard "white point". Our eyes regard that daylight colour as "white" or a neutral colour and TVs etc are calibrated to so that "100% red + 100% green + 100% blue" are in the correct proportion to make white = 6500K.

(If you've got a fancy TV, or even a not that fancy monitor, you'll find this "white point" in the settings, for people who like fiddling. The standard is 6500K though.)

And apparently if something got infinitely hot, it would be light blue. The line grinds to a halt.

Wow. That is so cool to learn. I'm actually supposed to be doing some continuing medical education modules right now but this is so much more interesting! Way back in high school I wanted to be a physicist but my math wasn't strong enough for hard sciences. Anytime you want to talk about this stuff I'm here for it!