Lots, it turns out. Our hormonal levels influence facial shape, for starters. And when we eyeball members of the opposite sex, we interpret their face shapes as indicators of attractiveness. Women whose faces indicate higher levels of estrogen are rated, by men, as better-looking.

It goes the other way as well. This story in The Economist looks at research into how women react to the shape of men’s faces — high testosterone-y, “rugged” faces vs. lower testosterone, softer features.

And then there is our response when we’re around someone attractive. Some researchers speculate that humans evolved the ability to see color because doing so helped us detect each others’ blushes.

What fascinates me most about all this is how it unconscious it is. We walk around with our big brains, interpreting everything abstractly. Meanwhile so many of our choices are influenced, if not governed by chemical signals, tiny variations in the size of facial features, tiny changes in skin color or pupil size.

We live double lives, and we don’t even know it.