I discovered that the guy I'm dating has a girlfriend he's cheating on. In fact, she confronted us, which was awful. I've had a history of guys cheating on me, and I want to end the pattern. Unfortunately, I'm not attracted to a lot of guys I go out with, and I'm really attracted to this guy.

--Chemistry

Being really attracted to somebody is the go-to excuse for shrugging off a potential partner's shady behavior -- pretty much anything this side of "Well, sure, several of his neighbors are wintering in Ziploc in his basement freezer."

It helps that attraction seems like some mysterious and magical force. It's actually not. Who we're attracted to -- which people, with which features -- is prearranged by our biological robot overlord, aka evolution, via genetic code written into each of us. It's part of evolution's scheme for building better babies -- giving our genes the best shot at being passed on to future generations.

We see this in research by neuropsychologist Bruno Laeng that suggests we are attracted to potential partners who look like us -- though not too much like us. Laeng found that people were most attracted to individuals who share about 22% of their facial features (as opposed to 11% or 33%, the other percentages tested). Other research by social-personality psychologists R. Chris Fraley and Michael J. Marks likewise hit the 22% mark.

Laeng explains that this balancing of "similarity and dissimilarity" (which we do subconsciously) helps us avoid "inbreeding with close relatives," like siblings or first cousins. Inbreeding increases the chances that both partners would have the same nasty recessive genes for a disorder or disease. "Recessive" genes are true to label when they are paired with a dominant gene: They recede...slumping into the background, unemployed, inactive. But when two recessive genes for a condition are paired (like when close relatives with the same recessive gene make a baby), these genes become active -- and so does the disorder or the disease.

As for you, the features you happen to be attracted to come in the package of a guy who cheats on his girlfriend. This reflects bad character. Assuming you didn't go out into Datingland all, "I've just gotta find a sexy sociopath," reflecting on the evolutionary nuts and bolts of attraction might help you stop using it as an excuse and give the shove to Mr. Morally At Leisure.

To avoid again letting the hots for some himbo blind you to his undesirable qualities, make a short checklist -- what I call "man minimums," the qualities you can't do without in a man -- and put character at the top of your list. When a man shows you he comes up short on your "must haves," cut off contact and move on. Ideally, if you're screaming in bed, it isn't because the girlfriend of the guy you're with just burst through the door brandishing a missile launcher.

For pages and pages of "science-help" from me, buy my latest book, "Unf*ckology: A Field Guide to Living with Guts and Confidence." It lays out the PROCESS of transforming to live w/confidence.