Evolution as I understand it favors selection of traits giving a species the best chance of reproductive success. Mates who are healthy, strong, faithful, hard-working, generous, and so on seem more apt to produce offspring who survive till reproductive age than those who aren't. However, physical beauty doesn't correlate with any of those qualities. So why have we evolved such a strong instinct to mate with those who look good and an aversion to those who don't?
"Babe," I said to Ms. Adams, looking up from a pile of scientific studies, "research suggests the reason I find you attractive is you have a low waist-to-hip ratio."
"So," said Ms. Adams, "you're saying my butt looks big?"
"Not at all. Scientists say, and this is close to an exact quote, a curvaceous body corresponds to the optimal fat distribution for high fertility."
"There's a line you'd never want to use in a lesbian biker bar," Ms. Adams said.
"Never mind," I said. "What are your waist and hip measurements?"
Ms. Adams got out the tape measure. "My waist is 27 inches and my hips are 37."
"There you go. That's an 0.73 ratio. According to research, the optimal ratio is 0.7."
"So I'm fat."
"No, you're optimal."
Ms. Adams punched up the calculator on her phone. "To be so-called optimal I'd have to have a twenty-five-and-seven-eighths-inch waist. I'd be anorexic."
"You're the opposite of anorexic. Your body fat is ... let me rephrase that. Your adipose tissue is advantageously arrayed, and I think the idea partly is that wide hips make it easier to give birth."
"So we're back to my having a big butt."
"Will you quit worrying about your butt? Other researchers say waist-to-hip ratio is overrated, anyway. They claim the real determinant of attractiveness is body mass index, or BMI, which is basically your weight scaled to your height. What's your weight these days?"
"OK, we convert that to kilograms, and divide that by the square of your height in meters, and ... oh, my."
"What?" Ms. Adams asked.
"Look here. Your BMI is 19.6. This chart plots the attractiveness of various female silhouettes as rated by 40 male undergraduates in Newcastle-upon-Tyne, England. The attractiveness curve on the BMI scale peaks at almost exactly 19.6."
"Huh," she said, glancing at the chart. "If I'm ever in the market again, I better move to Newcastle-upon-Tyne."
"The point is," I continued, "BMI corresponds with health, and health is closely related to fertility. The ideal BMI for general health is somewhere between 18.5 and 25, and women with BMIs from 20 to 25 have the lowest level of irregular periods. The chart suggests that, despite the media's fixation on impossibly thin models and actresses, men prefer women at the skinny end of the fertile range. Furthermore, if you look at this other chart, which plots attractiveness ratings in 3-D as a function of both BMI and waist-to-hip ratio, we see that the women rated highest of all had a BMI of 19 to 20 and a WHR of 0.7, which I can't help observing is pretty close to what you are. But the waist-to-hip thing was clearly a secondary factor.
"In other words, when I spotted you in shorts that one day, my subconscious thought was, mmm, fertile. Then I checked out your butt and thought, ooh, excellent arrangement of reserve energy deposits, will have many babies. And that sealed the deal."
Ms. Adams gave me a look. "It's lucky for men they keep their subconscious thoughts to themselves," she said. "If they were smart they'd always do that."
"We can learn so much from science," I went on. "Here's another paper suggesting a woman's body movements, scent, and so on vary depending on what time of the month it is, and that men subconsciously pick up on this and feel a greater or lesser degree of attraction. Proof of this is that strippers report higher lap-dance earnings on peak fertility days."
"Lap dancing?" Ms. Adams said. "Somebody got a grant to study this?"
"Babe," I said, "we must follow the quest for knowledge wherever it leads us. Think of the insight science has given us into the human condition. As a result of millions of years of evolution involving differential reproductive success, every so often I look at you, as male hominids have looked at their life partners since time immemorial. Maybe I get a whiff. And I think, whoa, you look really hot tonight — you must be ovulating."
The door slammed. Ms. Adams had left the room, leaving me with an urgent parting wish. But I don't think she meant what she said.
Send questions to Cecil via straightdope.com or write him c/o Chicago Reader, 11 E. Illinois, Chicago 60611. Subscribe to the Straight Dope podcast at the iTunes Store.
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