The Billy Joel Syndrome is one of the great social science mysteries: Why do beautiful women sometimes fall for not-so-good-looking guys?

For the answer, says Dr. Lee Alan Dugatkin, look to guppies. 

Guppies are the first species in which scientists have observed the phenomenon of "mate copying": A female guppy is much more likely to have the hots for a male guppy if other female guppies like him, too. And it seems humans behave in much the same way. 

Dugatkin conducted a study of 60 men and 74 women who were shown pictures of strangers and asked if they'd like to date them. They were also given information about whether other men and women, respectively, found the strangers attractive. 

"Unsurprisingly, the male and female subjects both liked attractive people," says Dugatkin, a professor at the University of Louisville. "But females weighed the opinions of other females much more highly than the men did." 

Women also described the guys they preferred in the pictures as more wealthy, funny and personable — even though they had never met them. 

"When humans see their peers flock around someone, they may wonder what the fuss is all about," the study says. 

In other words, Dugatkin says, mate copying "could lead a female to go out with a physically unattractive male because other females said they'd go out with him." After all, he must have something going for him. Right? 

Crudely put, Dugatkin's research suggests if you go out even once with an exceptionally attractive babe — say, Christie Brinkley, Joel's ex-wife — you may be set for life in the romance department. 

It's not that women are lemmings when it comes to getting a date. Rather, their behavior is strongly rooted in evolutionary decision-making. 

"Using other females' information is an expression of women being choosy," Dugatkin says. "From an evolutionary perspective, females should be the choosier sex; they have more to lose. If a female makes a mistake and chooses the wrong mate, she'll be out of commission for nine months. And if one parent leaves, it's almost always the male." 

So if a woman doesn't have much information to go on when sizing up potential mates, she'll probably prefer the guys that other women want to get busy with. 

Back to Billy for a minute. Since he and the Uptown Girl divorced, how has he done in the romance department? 

Pretty well. His current relationship is with local television news anchor Trish Bergin, who would rate pretty high on most men's attractiveness scales. Oh, and by the way: Joel was introduced to Bergin by Bergin's very good friend, Christie Brinkley.