Indiana University School of Medicine's Bill Sullivan describes ways our DNA influences behavior.

What makes one person like broccoli and another hate it? Is there such a thing as chemistry between two people? What makes someone lean left politically? Your DNA has plenty to do with it, according to the book, Pleased to Meet Me: Genes, Germs, and the Curious Forces That Make Us Who We Are, by Bill Sullivan, professor of pharmacology and toxicology, and microbiology and immunology, at the Indiana University School of Medicine.

Sullivan, who earned his doctorate from the University of Pennsylvania, recently joined the Knowledge at Wharton radio show on SiriusXM to discuss how much of our genes surprisingly contribute to our behavior. (Listen to the podcast at the top of this page.)

An edited transcript of the conversation follows.

Knowledge at Wharton: There has been a decent amount of research into the relationship between genes and traits, but why haven’t we reached a better understanding of it?

Bill Sullivan: I think a lot of the research outside of DNA is actually quite new. Even some of the discovery with genes and our behavior is on the cutting edge because we’ve long appreciated that they dictate physical traits, but we haven’t appreciated how far genes can really impact personality and behavior, in combination with forces in the environment, which introduce the new science of epigenetics.

Knowledge at Wharton: You call them “hidden forces,” referring to inside the body. Can you talk about that?

Sullivan: We usually think of things that govern the self — our personality, our actions — those are fully controlled by some sort of entity in the mind. But we are finding that we are kind of like Pinocchio, and there are a lot of strings attached to us. Some of those are genes. Some of those are epigenetics. Some of those are even the microbes that reside within our gut, and they’re pulling us in ways that we don’t realize, that we’re not fully conscious of.

When I got my Ph.D. at Penn, I worked in the laboratory of David Roos, and we studied this fascinating parasite called toxoplasma gondii, which amazingly is present in one-third of the world’s population. Most people don’t even know that they have it. There are emerging studies that this parasite, which lives in the brain for the rest of the person’s life, can be influencing certain neuropsychoses, everything from schizophrenia to rage disorder to risk-taking behavior.

Knowledge at Wharton: Obviously, we get our genes from our parents, which influence us in many ways. What about the influence of our environment or life experiences?

Sullivan: We are born with the genes that our parents gave us, and there’s nothing we can do about that, really. It’s kind of like sitting at the poker table. You get a hand of cards, and those are the ones that you have to play. But what we have to realize is that there are multiple ways to play that hand, and that’s where the environment comes in.

“Genes clearly make some people more susceptible to food or drug addiction.”

We can’t take someone’s genetic sequence and view it as a crystal ball — it’s not going to tell us everything about complex behaviors like personality traits and what people are going to do with their lives. Epigenetics is the study of how the genes that you have — the ones that mom and dad gave you — can be modified by factors in the environment. We’re not talking about changes in the DNA sequence, but we’re talking about modifications to the DNA molecule itself or the proteins that associate with it that can change how the gene is activated. It can turn it up or turn it down, much like a volume knob.

Knowledge at Wharton: How frequently does that occur?

Sullivan: This is a very active area of investigation for scientists. It seems to be happening even before conception. There are activities that men and women can engage in, such as their diet, whether they smoke or not, whether they consume a lot of alcohol. Epigenetic changes are already taking place in the sperm and egg of those individuals who are engaging in these behaviors. What that exactly means in terms of fetal development remains to be seen.

When the fetus is in utero, there are epigenetic changes that can be occurring there, too, based on the environment that mom finds herself in. They actually occur throughout the rest of our lives because there are things we encounter in the environment that can modify whether our genes get turned on or off or somewhere in between.

Knowledge at Wharton: How is addictive behavior influenced by our genes?

Sullivan: Many people think that conditions like addiction to drugs or alcohol, or addictions to food, which can lead to obesity, are self-inflicted. It’s a failure of willpower. But the science shows that it’s a little more complicated than that. Genes clearly make some people more susceptible to food or drug addiction, so it’s not simply a matter of self-control. Some people have a thrifty metabolism, and these genes are maladaptive in today’s world because it’s so calorie-rich and sweets are easy to find. Some people even have a gene for a sweet tooth.

In terms of alcoholism, some people have genes that build hyperactive brains, so they’re in this state of over-excitability, and it provokes them to use alcohol to quiet the chaos in their head, which can lead to alcoholism. There’s even evidence that our microbiome, all the trillions of bacteria that reside in our gut, can influence cravings for junk food or even the ability to stay sober. We can treat conditions like this much more effectively by learning the biology behind them, rather than just simply dismissing it as a lifestyle choice.

Knowledge at Wharton: Is willpower overrated?

Sullivan: Yes. I certainly don’t mean to dismiss it out of hand. It’s a component of all of this, and you need strong support groups, and change is certainly possible. We’re not trying to set up biological determinism here, but what we are saying is that we shouldn’t be too quick to dismiss someone who is struggling with these problems because it’s not a simple matter of willpower. These genetic forces, these microbial forces can be extremely potent. If we understand the nuts and bolts behind that at a molecular level, then we can come in and introduce new therapies that may be incredibly useful.

Knowledge at Wharton: Would that include long-term changes to how we eat rather than temporarily dieting?

Sullivan: Mom and dad have always said, “You are what you eat,” right? It sounds boring and trivial, but that is so true. Unfortunately, the Western diet is probably one of the worst things that we can do for ourselves. Just the extraordinary amount of sugar, fat and salt that we consume on a regular basis is a far cry from what our bodies were evolved to digest on the African plains. That’s why we have these sweet tooths and this drive towards fat. Those are calorie-rich, and calories were hard to come by back on the African plains. We evolved bodies to be pretty thrifty with regard to metabolism, and now that we’re surrounded by all these calories, it’s having horrible effects in terms of obesity and diabetes.

“There really is something like chemistry that can draw two people together.”

So, we have to take control of the microbes in our gut, and our food can be a really important source for that because our microbes produce substances that make us hungry, that make us crave the nutrients that they need. There are certain species of bacteria that thrive on sugar, or species that thrive on fat, and they’re going to make you hunger for those materials. You get caught in this vicious feedback loop.

Knowledge at Wharton: But we see the differences in what we crave even here within our own country, within our own culture, from person to person.

Sullivan: Exactly. That’s where we can come back to genes. One of the things that prompted me to write this book is that our genes can dictate what sorts of food and drinks we like or dislike. That’s pretty powerful because we think of our preferences of food and drink to be one of our most self-defining qualities, and here it is, like a puppet string — genes are influencing us to like certain foods.

For example, I can’t stand broccoli. I can’t stomach it. I’ve been that way all my life. My daughter is like that. I was wondering all throughout my life, “What in the world is wrong with me? I see all these other people putting broccoli into their mouths, and they’re seemingly enjoying it.” I could never experience that. It turns out I have a genetic mutation that builds and puts taste bud receptors on our tongues. I got tested for this, so I have the data. I can show my parents, “Look, I wasn’t joking. This broccoli tasted extremely bitter to me because my tongue is filled with taste buds that bind to the bitter chemicals in broccoli.”

Knowledge at Wharton: Musician Ozzy Osbourne has said that he has this innate ability to drink a lot, use drugs and still have a relatively healthy body.

Sullivan: It’s pretty amazing. Science actually sought out Ozzy Osbourne back in 2010. This is when genome sequencing was just coming online, so it was very expensive and very laborious. You would think that scientists would go after maybe the most intelligent people in the world or the most artistic or the most athletic. To a degree, Ozzy is artistic. They actively sought him out because they were puzzled. Why is this guy still alive, after all he’s done to his body, after all that drug and alcohol addiction? What’s going on there? Maybe we can learn something about people who are sensitive to addiction in general, by looking at his genome. So, they sequenced it.

Knowledge at Wharton: Hopefully, they searched out Keith Richards of the Rolling Stones as well, because he could be in that same boat.

Sullivan: I joke that Keith Richards is not built from DNA. He is built from something entirely extraterrestrial. I don’t know what it could be, but that is a remarkable specimen, as well.

But they got DNA from Ozzy. He was all on board with this. He was fascinated by it, even though he joked that the only gene he knew anything about was the guy in KISS, referring to Gene Simmons. So scientists assembled what I call “The DNA Diary of a Madman,” stealing one of the titles of his albums, and they found a never-before-seen mutation in a gene called ADH4, which is interesting, because that’s an alcohol dehydrogenase that resides in the liver, and it detoxifies alcohol. The rest of the mechanism hasn’t been fully worked out, but scientists have speculated that because of that mutation, Ozzy might be better able to process alcohol, detoxify it more quickly.

“There could be a thread of our biology that tugs us towards one end of the political spectrum or the other.

… It’s probably not a coincidence that he’s got a variation in a critical liver enzyme that processes alcohol. He also had some variations in a variety of genes that encode for neurotransmitters and some other facets of the brain that make him six times more likely to crave alcohol in the first place. So he probably has one of these hyperactive brains that I talked about earlier, which is normally calmed down by a neurotransmitter called GABA, which looks very similar to alcohol. Alcohol is basically a form of self-medication to calm the brain down — a bad medication, but it calms the brain down.

Knowledge at Wharton: Let’s talk about looking for a romantic partner. How do genes factor into that?

Sullivan: There are some really fascinating studies, one of which relates to genes and another one which actually relates to the microbes in our gut, which is very surprising. When I started to research this, this was another topic that was kind of personal to me. I’ve withstood my fair share of rejection throughout my single years, and I was always wondering, “What’s wrong with me?” I feel much better about it [now]. People who read this book will probably feel better, too, because it’s more like organ rejection when someone turns you down. Let me explain that.

There was this famous experiment involving women sniffing sweaty T-shirts that had been worn by men. It doesn’t sound like something you want to do on a Friday night, but it was all for science, so it was for a good cause. But the interesting result was that women who had immune system genes that were very similar to the man, she found his scent repulsive. Whereas if the man had immune system genes that were different from hers, she was attracted to his smell. There really is something like chemistry that can draw two people together, and the whole reason behind this is evolutionary because when we procreate, we want offspring that are better equipped to fight off germs and pathogens. If you have a diverse immune system, the baby is going to have a higher probability of fighting off those diseases.

Knowledge at Wharton: Is there an element that can explain the high divorce rate?

Sullivan: Divorce rates are about 40% in this country, so that’s nothing to shake a stick at. But it’s also nothing to be embarrassed about. There has been a lot of pressure, especially in America, to stay together for life, and that’s really weird to do. It is extraordinarily rare that any other species does that, so we’re unusual in that regard. I’m not saying that people should stop doing it. If you can live with the same person until you’re 80 or 90, that’s fantastic. I hope to be one of those people.

Monogamy is very weird in terms of coupling in the animal kingdom. Helen Fisher wrote some fantastic books about this. She’s an anthropologist who has studied this probably more than anybody else on Earth. What we usually find in species that pair — and again, we’re only talking about like 5% of all the species in the world — is that they usually do so only for a certain number of years. And that number of years correlates directly to when the offspring can take care of itself.

Then the parents usually go their separate ways. Some of them stay together and have another child. That’s called serial monogamy. But others find different partners and diversify their genetic portfolio. There are pros and cons to each side. You get genetic diversity if you have multiple partners, but you also get family units, which can be tremendous strengths, if you stay with the same individual.

Knowledge at Wharton: You also talk in the book about people’s beliefs. Much of what we believe is passed down from our parents and family members. But political beliefs can start to separate people a little bit, maybe because of outside influences.

Sullivan: It certainly can. This is a realm of science that’s called genopolitics. Certain researchers have looked at people on the far right and the far left and tried to see what kinds of genetic differences exist between the two, if any.

One of the most interesting genes that have fallen out of these studies so far is a variation in something called DRD4. This is a gene that encodes a dopamine receptor. Dopamine is a critical neurotransmitter in the brain that drives us towards rewards, and it is variable among the human population.

Some people have a variation of this receptor that causes them to engage in more risk-taking and novelty-seeking behavior. They’re more exploratory. That variation tends to be associated with liberals. I found that to be very fascinating, that there could be a thread of our biology that tugs us towards one end of the political spectrum or the other.

“Our brain loves to hear when people agree with us. You see a flash of dopamine associated with this.”

Knowledge at Wharton: I want to finish up by asking about social media. How does the digital culture impact us?

Sullivan: It certainly impacts us on a number of levels. It can affect your mood, for one thing. Facebook, unbeknownst to its users, conducted a study several years ago where they selected certain members and then just started putting a whole bunch of either negative posts or positive posts on their page. Again, the user was oblivious to this. They were not informed.

It turns out that people who were flooded with negative posts started posting more negative things. People flooded with positive posts started posting positive things. So, we can certainly be influenced at a subconscious level based on what we’re reading in social media. We need to be vigilant about that.

Another important point about social media is that our brain loves to hear when people agree with us. You see a flash of dopamine associated with this. Our brain eats it up like candy. Social media makes it incredibly easy to generate echo chambers so that all we do is preach to the choir all the time and disparage the other side, without really getting to know them or appreciate the merits that might be in their arguments. It’s important that we step outside of these echo chambers once in a while and meet people who are different than you.

I dedicated sections of each chapter on how we can utilize this knowledge or power because I believe it can be an important agent for change. By exposing these hidden forces, we’ll get to know ourselves better, and just as important, we’ll get to understand people who are different than us.