[intro music]

Anyone, or, or anything that reproduces sexually is constantly thinking to itself, "Who am I gonna mate with?"  We spend a lot of time thinking about mate choice because passing on your genes early and often is how you win at being alive.  From an evolutionary standpoint, it's the most important thing you will ever do, if you do it.  And due to its extreme importance, it's also very complicated, you may have noticed.

Our general understanding of it comes from Charles Darwin who proposed that some individuals get more action than others because of sexual selection.  You've heard of it; one gender of a species consistently prefers mates with specific traits, so individuals with those traits pass their genes on more often and spread their genes and the traits throughout a population.

Old Charlie was certainly onto something; take a look at this peacock.  Some would consider this...overkill, but, to the ladies, a big frothy rack of feathers shows that a dude is healthy and has good genes.  So, as a result of sexual selection, male peacocks have evolved to look like 1970's Las Vegas showgirls.

Humans, like peacocks, have evolved to like some traits more than others, and these preferences date back to a time when those traits gave an individual huge survival advantages.  So, in most cases, beauty is actually a language that conveys information about health and fertility.

But if you're thinking that it's all about boobs and big muscles, think harder.  You know what really turns us on?  Bilateral symmetry.  Humans are attracted to people whose bodies and faces look the same on both sides.
How am I doing?  Anybody?
Symmetrical imbalance can mean someone was exposed to some kind of stressor during early development that they weren't tough enough to deal with.

Similarly, a low waist-to-hip ratio drives both dude and lady humans wild.  Where fat is deposited on a body is determined by sex hormones: testosterone in men and estrogen in women.
Women who produce the ideal amount of estrogen generally have waists that are about 70% the width of their hips.  Same goes for men and testosterone.
People with this ideal ratio are often less susceptible to cardiovascular disorders, cancer, diabetes, and women in this range also have less difficulty conceiving and giving birth.

Now, you might be wondering, why will an isometrical guy with an ideal waist-hip ratio make you feel kind of, meh, but he's got your friend driving by his house three times a day?
Well, one reason could be in our genes.  Everybody gives off subtle chemical signals called pheromones which give potential mates a clue to our genetic profiles.
Crazy, right?  Studies have shown that men and women are generally attracted to pheromones of individuals that are genetically similar to themselves, with one major exception.  They prefer mates with a different kind of immune system gene, called Major histocompatibility Complex, or MHC.  The combo of two variations of the MHC gene helps a couple's offspring fight off disease better and our pheromones basically tell mates which version of that gene we have.

But even after you account for pheromones, and symmetry, and ratios, it's complicated, and people don't make rational decisions.  So really, don't bother with those hair plugs or that boob job; it's complicated.  And yet so awesome.
Thanks for watching this episode of SciShow.  In the comments, tell us, uh, what makes you attracted to a potential mate.  Also, if you have any questions or ideas or comments you can leave those down in the comments or get in touch with us on Facebook or Twitter, and if you want to continue getting smarter with us all the time you can go to youtube.com/scishow and subscribe.