Did you think the concept of "opposites attract" is just way to explain how Type As ended up with Type Bs? In one study, women were given smelly T-shirts of men (nice experiment, eh?) and told to pick the ones they found most attractive. You know what they picked? Not the Iron Maiden Ts, but the ones of people whose genes were most different from theirs, indicating that we're actually hard-wired to diversify the gene pool. This also keeps siblings from being attracted to each other, which (besides being weird) increases the chances of genetic defects.
Find out more about this book:You: Being Beautiful - The Owner's Manual to Inner and Outer Beauty