What scares you the most, a rattlesnake or a car? Most likely it's a rattlesnake, although common sense should tell us that we're far more likely to die from an encounter with a car than a snake.
The reason why we fear reptiles more than cars is a cornerstone of the relatively new field of evolutionary psychology, and it helps explain how we became who we are today. It also tells us much about ourselves and our fears and emotions and cravings, and why, for example we can't seem to push back from the dinner table when we've already had enough to eat.
The mechanism that created our fear of snakes also left us with cravings that help explain why so many folks are just too fat, experts say.
It's all in our genes, and the lives our ancestors lived a relatively few generations ago. In fact, if they had not had many of the same cravings we have today, we might not be here at all. But some of those old traits are coming back to haunt us.
"Basically, we're living in a world that's not the world we evolved to function in," said anthropologist Dan Fessler, director of the Center for Behavior, Evolution and Culture at the University of California, Los Angeles.
Fessler is a lead player in the effort to understand how the world in which our ancestors evolved forced them to avoid some things and hoard others just to survive, and how those ancient needs impact us today.
"The basic logic is that natural selection can shape the mind to shape behavior," Fessler said.
Fessler's primary area of research is an emotion that we all feel from time to time: disgust. But he sees it differently than the rest of us. According to his research, disgust helped our ancestors reproduce in a world filled with pathogens.
"The emotion allowed our ancestors to survive long enough to produce offspring, who in turn passed the same sensitivities on to us," he said.
To prove his point, Fessler has conducted a number of clever studies of pregnant women. The experiments were designed to show how disgust can protect a woman during her most vulnerable times, like the first trimester of pregnancy when the woman's immune system is suppressed to keep her from rejecting the infusion of new genetic material in her womb. Since disease is generally passed from one person to another through contact, Fessler reasoned that women would find some things particularly disgusting during that critical first trimester.
So he asked them to rate their level of disgust over being exposed to scenarios like seeing someone stick a fishhook through a finger, or seeing "maggots on a piece of meat in an outdoor garbage pail." As expected, women in the first trimester scored much higher in disgust sensitivity, particularly when the scenario increased the possibility of contact with others or eating contaminated food.
Disgust, his research shows, played a central role in human procreation, and the same passions continue today.
Fessler notes that our species, defined as people whose bodies looked like we look today, has been around for at least 150,000 years, and probably much longer. For nearly all of that time we were hunter-gatherers. Only in the last few 1,000 years have we developed agriculture, and domesticated animals, allowing us to build cities and stick around the same territory.