I know it's the first thing I look for...

Why are we attracted to some members of the opposite sex? You are not going to believe this, but it may be because of immune system compatibility.

So much for outward appearances. What about the less obvious cues of attraction? Fascinating work on genetics and mate preferences has shown that each of us will be attracted to people who possess a particular set of genes, known as the major histocompatibility complex (MHC), which plays a critical role in the ability to fight pathogens. Mates with dissimilar MHC genes produce healthier offspring with broad immune systems. And the evidence shows that we are inclined to choose people who suit us in this way: couples tend to be less similar in their MHC than if they had been paired randomly.

The article also offers some of the most nerdy dating advice I've ever run across:

The researchers found that the optimum proportion of possible mates to “examine” before setting your aspirations and making your choice is a mere 9% — so at a party with 100 possible mates, it’s best to study only the first nine you randomly encounter before you choose. Examining fewer means you won’t have enough information to make a good choice, examining more makes it likely you’ll pass the best mate by. No doubt the models underestimate the complexity of real mate choice, but the fundamental insight is clear: don’t search indefinitely before choosing, lest you miss out on all the good mates or run out of time altogether.

Not that I'm interested in that kind of stuff, of course.

I'm married. Always have been, I think.