Introduction to Human Behavioral Biology

A student in Beijing walks into a coffee shop to meet a friend. Two chairs block the path between the tables. There are two ways through: walk around them, or pull the chairs apart.

Here is the strange part. If the student's ancestors were rice farmers, they are more likely to walk around. If their ancestors were wheat farmers, they are more likely to shove the chairs aside.

Nobody taught them that. It comes down to the crop their ancestors farmed.

• Rice: growing it means flooding shared fields and working with your neighbors, so rice-farming cultures grew up valuing teamwork.
• Wheat: growing it is a job you can do alone, so wheat-farming cultures grew up valuing doing things yourself.

A crop your ancestors grew centuries ago is still nudging a small decision made in a coffee shop today.

Five Signs Something Deeper Is Steering You

That coffee shop finding is one of five that frame this whole subject. Each one points at something working underneath conscious choice.

• A child's guess. Show five-year-olds two photos of adults and ask them to pick the "captain" for a pretend trip to Candy Island. The photos are secretly of two politicians who once ran against each other. Average out the kids' answers, and they pick the real election's winner about 70% of the time.
• A hormone and a name. In the classic trolley problem, you can divert a runaway trolley to save five people, but it will kill one person tied to the tracks. Raise people's oxytocin, the hormone linked to bonding, and their choice shifts. Give the person on the tracks a name from your own group, and oxytocin makes people less willing to sacrifice them. Give them a name from a rival group, and oxytocin makes people more willing to let them die.
• A womb before birth. A fetus has no say in whose womb it grows in. By the third trimester, a mother's poverty already shows up in the data: babies developing in poverty tend to have smaller brain volume than average.
• A face on a given morning. Show women photos of men's faces at different points in their monthly cycle. Near ovulation, when estrogen peaks, women lean toward rugged, strong-jawed faces. Later, as progesterone rises, preference shifts toward softer, kinder-looking faces.

A coffee shop, a kids' game, a moral dilemma, a womb, a face. Five different settings, and in every one, biology beneath awareness is doing real work.

Biology on Trial: Four Defenses That Worked

Take four seemingly unrelated things: a menstrual cycle, a brain tumor, a diet of junk food, and anabolic steroids. All four have been argued in real courtrooms as reasons to reduce a sentence for violent crime.

• The menstrual cycle. A woman's documented history of severe mood changes tied to her cycle helped lower her sentence in a murder case in England.
• The amygdala tumor. The amygdala is a small part of the brain that handles fear and threat. A tumor pressing on it has been accepted in court as the real cause of sudden, out-of-character violence.
• The "Twinkie defense." After Dan White killed San Francisco Mayor George Moscone and Supervisor Harvey Milk in 1978, his lawyers argued that eating too much sugary junk food caused wild swings in his blood sugar, and that this threw off his behavior. It helped him get a lighter sentence.
• Anabolic steroids. Large doses of man-made testosterone, sometimes called "roid rage," have been used in court to argue that a person was less in control of their own aggression.

The common thread: what happens in your body, your hormones, a tumor, your blood sugar, can change what happens in your brain, and therefore what you do. That is the first of two ideas this whole field rests on.

The Two Threads of This Course

Here is the second idea, and you can test it on yourself tonight. Lie in bed, relaxed, listening to your own heartbeat. Now deliberately think about the fact that this heart will someday stop, picture the last beat, picture your blood going still.

You have not moved a muscle, but your breathing will speed up and stress hormones will start flowing. A thought alone, with no exertion and no injury, just changed your body.

Put both ideas together and you get the two threads this entire field is built on:

• Body → Brain: what happens in your body, hormones, tumors, food, changes how your brain works.
• Brain → Body: what happens in your brain, thoughts, memories, feelings, changes how your body works.

Out of that loop comes everything we mean by "mind."

Why "It's Complicated" Is the Only Honest Answer

If there is one idea to hold onto through all of this, it is that the honest answer is almost always "it's complicated." Not "this causes that," but "this makes that more or less likely, in several different ways, and most of those ways have exceptions."

That is already true for simpler questions in biology, like why every wildebeest on the African plains comes into heat in the same week each spring. Or how a bird finds its way from the tropics all the way to the Arctic with no map. Those are genuinely hard problems, and science has made real progress on them.

Human social behavior is a harder problem still. It is the same body-and-brain loop, but with culture and history piled on top of it, and behavior that one person calls heroic and another calls terrible. The complexity does not go away here. It gets bigger.

Faced with something this tangled, the mind reaches for its usual shortcut: break the hard thing into smaller, labeled pieces, and hand each piece to a specialist. That shortcut helps memory, but it also quietly distorts how we see the whole picture, and it is the exact trap this course is built to avoid at every step.

Further Reading

• Book: Behave: The Biology of Humans at Our Best and Worst by Robert Sapolsky, the book-length companion to this lecture series.
• Wikipedia: Robert Sapolsky · Oxytocin · Trolley problem · Twinkie defense