Evolution and Behavior: Fear, Aggression, and Overeating

From a lecture series presented by The Great Courses

When we look at our proclivity for violence, and at the obesity epidemic, it seems like humans are bringing about their own destruction. But evolution and behavior are closely intertwined, and these tendencies were once paradoxically linked to survival. Do they still serve us in the modern age, though?

Two dangerous lions fighting aggressive showing teeth and scared fear

Origins of Irrational Fear

Imagine getting into a car to go somewhere. Think about it. How frightened or uneasy would you be as the car starts to move down the street? The question may seem like a strange one, but consider it: on a scale of 1 to 10, 1 being the least scared and 10 being the most scared, how scared or nervous would you feel to be in a moving car? Many people would likely say “1” on that scale, or maybe “2,” at most.

Now, imagine sitting by yourself on a park bench somewhere. Do you have the image? You’re sitting on the bench. And you look over, and maybe five feet from where you are sitting is a snake. It’s not doing anything—it’s just lying there looking at you. How uneasy or frightened would you be? On a scale of 1 to 10, how scared or nervous would you be to find yourself sitting next to a snake?

People will differ on their comfort level with the situation; some people may be more comfortable with snakes than other people. But even so, it’s doubtful that there are many people who would rate their degree of nervousness as only a “1.” For some people, their level of anxiety would likely be much closer to a “10.”

This is a transcript from the video series Understanding the Mysteries of Human Behavior. Watch it now, on The Great Courses Plus.

The number of people killed in automobile accidents far exceeds the number of people killed by snake bites. For the past several years, automobile accidents have caused between 35,000 and 40,000 deaths, and about 3 million injuries, each year in the United States. That’s an average of more than 100 deaths and 8,000 injuries per day. According to available records in recent times, no more than 12 deaths have resulted from snake bites.

By all rational standards, you should be much, much, much more afraid to get in your car than to see a snake. But you’re not. This irrational fear is where evolution comes into play. During the course of human evolution, while we were wandering around on the African savannah, snake bites were a real threat.

Our prehistoric ancestors who were afraid of snakes were much more likely to survive and reproduce than those who weren’t afraid of snakes; or worse, those who tried to play with snakes.

Learn more about behavioral adaptations

Evolutionary Function of Aggression

Another puzzling behavior that might have some evolutionary basis involves aggression. Human beings are a very aggressive species. Whether we’re talking about large scale instances of aggression such as genocide, aggression between countries, street violence, murder, or domestic abuse, human beings inflict a great deal of pain on one another. And even if we don’t actually harm other people, most of us occasionally have aggressive urges that we have to hold in check.

Most aggression seems somewhat pointless. Countries go to war over rather minor disputes, and people kill others for very minor slights. Little is often required to unleash a violent episode. Clearly, the tendency for aggression is built into the human psyche. But why?

According to evolutionary thinking, many instances of modern-day aggression are manifestations of strategies that facilitated survival and reproduction during evolutionary history. Looking at other animals, it’s easy to see how aggression helps animals protect their territory, defend themselves and their offspring, and obtain and protect resources such as food.

The same is true of human beings. In the days before we had culture and law enforcement and armies to protect us, each person had to be able to fend off threats with aggression when necessary. Nonaggressive individuals simply would not have survived at the same rate, and their offspring would have been at risk, as well.

Of course, in modern civilizations, personal aggression is rarely helpful and, in fact, it can get a person into a great deal of trouble. But features of human nature evolved because they were adaptive in the environment in which our prehistoric ancestors lived during human evolution, and they may or may not make sense in the environments in which we live today.

Human nature evolved mostly on the plains of Africa back when our ancestors wandered around as nomadic scavengers, gatherers, and hunters, long before there were settled communities.

Some of the most ornate anthropomorph petroglyphs by the ancient Fremont people, including warriors and severed heads on Mcconckie Ranch in Dry Fork Canyon, near Vernal, Utah, USA
Unlike in today’s world, aggression in early humans was necessary for survival.

If we go back to the point where the genetic lineage that led to modern human beings split off from the lineage that led to our closest modern relatives—the chimpanzees and bonobos—we’re talking a span of over six million years or more.

Of that six million years, only in the past 10,000 years have we had anything resembling civilization, with agriculture and settled communities. But biologists largely agree that our brains have not changed much, if at all, in the short span of time that human beings have been living in civilized communities.

Think about what that means: We have essentially the same brain that our prehistoric ancestors had during the Stone Age. We are literally living in the modern world with a Stone Age brain. And this fact helps to explain behaviors that are part of human nature but that are not beneficial to many of us today. In modern society, outbursts of deadly aggression are rarely beneficial to anyone—certainly not to the victim and usually not to the perpetrator. Everybody loses.

But back before culture, and police forces, and a judicial system, individuals who were willing to resort to lethal aggression to defend themselves, their food, their offspring, their clan, and their territory were more likely to survive—and to have children who survived—than individuals who were not aggressive. A willingness to resort to aggression was essential.

Today, we have police officers and home security systems and the courts to help protect us. Most aggression seems rather absurd today, but not too long ago it was essential.

The Roots of Overeating

Another example of an evolutionary adaptation that creates problems today involves the physiological mechanisms that control our eating behavior. There’s an obesity epidemic in many countries, including the United States. Looking at all of the health problems associated with being overweight, it looks like our evolved eating mechanisms are malfunctioning.

Why do so many of us eat too much, and even continue to eat when we aren’t hungry, and even when we consciously know that we’re undermining our health and shortening our lives?

Overweight woman eating junk food.
Humans tend to eat whenever food is available, whether hungry or not.

The answer seems to be that human beings have an evolved mechanism to eat whenever food is available. During virtually all of human evolution, our days were spent looking for food. When we found some, it made a great deal of sense to eat as much as possible because we had no way to store or carry extra food, and we didn’t know how long it might be until we would eat again.

The problem is that, in developed countries today, most people now have much more food available than they need. We can go out and eat as much we want, 24 hours a day.

To make matters worse, we seem to have a particular penchant for sweet and fatty foods. We rarely hear of people eating too much lettuce or celery, or too many tomatoes. The answer might be because those sorts of foods were fairly common on the plains of Africa, so there wasn’t any benefit in being a glutton when it came to vegetables. But sweet fruits were less common, and fatty animal flesh even less common.

Again, when our prehistoric ancestors finally found sweet fruits or they killed an animal, it was advantageous for them to eat as much as possible. So, unless we work very hard to control ourselves, many of us happily eat a big, juicy burger, and then head off for dessert.

Learn more about why self-control is so hard

Evolutionary Psychology and Free Will

Many people react negatively to evolutionary explanations of behavior because they think that if some behavior—such as aggression or overeating—is natural, then it seems like we can’t change it or maybe we shouldn’t even try. But that’s not the case. True, these reactions were natural and normal in the environment in which evolution occurred, but that doesn’t mean that modern human beings should necessarily act in these ways.

None of us are living naturally according to our evolved urges. If we lived like prehistoric people, we wouldn’t build houses or wear clothing or practice sanitation or use birth control.

And just because human nature was designed a certain way to meet the challenges of prehistoric life doesn’t mean that we can’t consciously decide to behave otherwise. We can control our eating within limits, we can manage our aggressive impulses, and many of us can make ourselves pick up a snake.

Controlling a particular reaction is more difficult if it involves an evolutionary designed system, but that simply means that we have to try harder—not that we shouldn’t try at all. And knowing that we’re sometimes working against millions of years of evolution helps us to understand why our human nature sometimes leads us to behave in ways that aren’t always in our best interests.

From the lecture series Understanding the Mysteries of Human Behavior, taught by Professor Mark Leary

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