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.
Hunter-gatherers had to live in small groups and move around because "once you've caught all the bunnies you have to go someplace else and catch bunnies there," he said. "So we evolved to function in small groups that moved about the landscape to make a living hunting and gathering."
"As a consequence some of our predilections are actually detrimental. We really like the taste of fat and salt because those were rare and valuable for our ancestors but they kill us today because we have them in abundance."
Obesity, he says, is the simplest example of how some of our evolutionary traits have come back to haunt us. Our ancestors didn't live in a world in which they could "drive up and supersize," he said.
If you are a hunter-gatherer, he says, "you have to go out and catch that mammal and that's a lot of work. That means we are designed for a world in which you have to expend calories in order to get calories."
Sometimes it must have been really difficult.
"If you have to work hard to get calories and there are times when calories are extremely scarce, then individuals who have a set of preferences that lead them to maximize the correct food intake when food is available will have done better," he said. "They had fat reserves when times were lean that the other folks didn't have. So we are descended from the people who liked the taste of fat. The ones who didn't like the taste of fat didn't make it."
Thus we are evolutionarily predisposed to pig out.
Fat didn't kill our ancestors in their challenging environment, he says, and today we have no "mechanism that says you live in a world where food is cheap and easy to come by, there's more fat than you can use, so don't like it anymore. That's not the way it works."
The consequence can be deadly.
"Very soon dietary causes of death will overtake all other preventable causes of death in the United States," Fessler said. "Right now smoking still leads by a little, but probably within the next couple of years the rates of death due to dietary-related behavior, primarily obesity, will overtake smoking."
"So we're dying at an enormous rate basically from having the dietary preferences of ancestors who lived in a world where there was no McDonald's."