It's not just those three margaritas compelling you to tip more. It's the person who's bringing you them.

A new study published in the Journal of Economic Psychology found waitresses whose customers deemed them as attractive tended to tip more.

A lot more.

Over the course of a year, servers who diners considered more "strikingly beautiful" could expect to earn roughly $1,261 more in tips than a homelier server.

The weirdest part? Those tips were coming from women.

Beauty might be in the eye of the beholder, but biology certainly clouds the lens. Attractiveness has been a rich field of study for decades and a subject of philosophical inquiry for millenia.

We know, for instance, that men tend to prefer high cheekbones to low ones and full lips to thin ones. Women like men to have strong jaws and broad chins.

What's more, winners of the genetic lottery go on to enjoy numerous benefits as they move through life. Beautiful people are more persuasive, have more sexual and romantic partners, seem more trustworthy, and make more money — approximately 12% more, in fact.

Living in a predominantly heterosexual world, we might assume pretty women get ahead because men believe their arguments more, pursue them romantically, trust them, and pay them more.

At least in restaurants, men might not be so in control.

Matt Parrett, an economist at the Food and Drug Administration, carried out the latest study. After he and an assistant collected 501 surveys from diners that asked how big they tipped and how attractive they thought the server was, Parrett analyzed the data for patterns.

According to him, the result that female servers earn bigger tips from other women might be explained, at least partly, by the fact female customers discriminate more strongly than men do in terms of attractiveness.

"As to why beauty pays," he writes, "I considered three explanations – stereotypes, increased confidence and better negotiation/oral skills, and taste-based discrimination."

Those first two explanations fell off the table when Parrett conducted his analysis. People don't make a judgment on the server's productivity until after the meal, he explains, so stereotypes wouldn't factor in. Likewise, negotiation skills played a statistically insignificant role in the process.

The only factor Parrett couldn't rule out was the idea that more beautiful female servers simply compelled female patrons to open their wallets.

"I am unable to reject customer taste-based discrimination as an explanation of why beauty pays in this environment," he writes.

According to some evolutionary psychologists, the modern-day benefits of being beautiful — an extra three bucks on an order of chicken wings and beer — actually stem from older humans' desire to seek out people without harmful genetic mutations. Average, proportional faces gave humans this impression over highly unique, asymmetrical ones.

If someone's face demands less work of our brain's visual system, we tend to favor it, which means we'd rather send our genes into the future with that person than someone else.

As to why females, more than males, tend to reward attractive female servers, Parrett posits that empathy could be at play.

"Maybe women are rewarding other women for the effort they make in trying to look good," he writes, "something which women might know and care more about."

In any cases, bars should take notice: If you don't already have a ladies' night, start one.