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Human Behavioral Biology · Lecture 7 of 25 · 1:32:44

Lecture 7: Behavioral Genetics II

Study guide

What this lecture covers

This lecture continues the course's look at behavioral genetics, moving from the classical twin and adoption studies covered earlier to the molecular techniques used to actually find genes linked to behavior, and finishing with a deep unpacking of what a heritability number really means.

You'll come away able to describe how researchers narrow a trait down to a genetic marker and then to an actual gene, recognize several real examples of genes tied to social and emotional behavior, and correctly explain why a high heritability score does not mean a trait is fixed by genes.

Key ideas

Phenotype-first gene hunting: early behavioral genetics started with an observable trait difference (often a severe disease) and searched DNA for a shared stretch that tracked with it, producing a genetic marker rather than a single gene.
Genetic marker: a stretch of DNA containing many genes that is statistically associated with a trait, not yet the specific responsible gene.
Reverse strategy: starting from a gene already studied in animals (its function known), then checking whether humans have similar variants and whether those variants map onto behavior.
Gene-environment interaction: the finding that what a gene "does" depends on the environment the organism is in, so the effect of a gene can reverse or vanish depending on conditions.
Heritability: a statistic describing how much of the variability in a trait across a population is explained by genetic variation, not how much genes determine the average level of the trait.
Environment inflation bias: because scientists typically study a trait in one tightly controlled environment, standard experimental practice systematically overstates how heritable a trait appears to be.
Chance as a third factor: alongside genes and environment, random molecular events (such as Brownian motion during cell division) introduce variability that is neither inherited nor environmental.

Walkthrough

Recap and the shift to molecular genetics (3:03)

The lecture opens with a quick review of twin, adoption, and prenatal-environment studies from the prior lecture, then pivots to how, starting around the 1980s, researchers began combining classical behavioral genetics with molecular biology to actually locate genes. It walks through a ladder of starting points, from a clear phenotype difference down to differences in protein size, each used as a foothold for tracing genetic differences.

From genetic markers to disease genes (11:09)

Using diseases like PKU, Huntington's, and cystic fibrosis as early targets, the lecture explains how comparing DNA across affected and unaffected relatives in large families produced genetic markers, broad chromosomal neighborhoods rather than single genes. It covers the bioethics problems this raised, such as whether to test people for adult-onset diseases before symptoms appear, and how sequencing technology later let researchers pin down markers to specific genes and mutations.

Modern tools and finding genes tied to behavior (16:15)

The lecture introduces gene microarrays (credited to Stanford's Pat Brown) and quantitative trait loci analysis as tools for studying many genes at once rather than one at a time. It then flips the approach: starting from genes already understood in animal studies, such as the vasopressin receptor gene linked to monogamy in voles, and checking whether the same gene variants exist in humans and predict similar behaviors, including relationship stability and reading facial expressions. Similar examples follow for BDNF (linked to amygdala-driven anxiety), dopamine receptor D4 (linked to novelty-seeking), and neuropeptide Y (linked to anxiety and metabolism). Each of these genetic links is described as real but explaining only a small fraction of the variability in the trait.

What heritability actually measures (39:34)

This section reframes heritability as a measure of variability, not average trait level, and walks through the counterintuitive plant IQ example: if a gene fully explains differences in a trait within one environment, but the same gene shows no consistent effect once you add a second environment, heritability drops sharply. It shows how studying a trait in only one controlled environment (the standard, "careful" way to run an experiment) mechanically inflates the apparent role of genetics by removing environmental variability from the picture.

Counterintuitive cases and real interaction data (55:51)

Using the number of fingers on a hand (0% heritable, since variation is almost entirely from accidents) and 1950s earring-wearing (100% heritable, since it tracked sex almost perfectly), the lecture shows heritability can defy intuition entirely. It then covers real gene-environment interaction studies: PKU heritability dropping to near zero once diet is controlled, Caspi and colleagues' New Zealand study linking a serotonin-related gene to depression only in people with childhood stress exposure, and a parallel finding for a MAO gene variant and antisocial behavior tied to childhood abuse.

Environment overwhelming apparent genetic effects (1:22:11)

The lecture closes on gender differences in math and verbal test scores across 40 countries, showing that the size and even direction of these differences track a society's gender-equality index far more closely than they track sex itself, with the gap disappearing or reversing in more equal countries such as Iceland. This is used to reinforce that knowing someone's environment is often far more predictive than knowing their genotype.

Before you watch

Review the earlier lecture on classical behavioral genetics (twin and adoption studies) since this lecture assumes familiarity with monozygotic/dizygotic comparisons and epigenetics.
Be comfortable with the basic idea of a gene having different versions (alleles) and a promoter region controlling gene expression.

Check your understanding

What is the difference between a genetic marker and an identified gene, and why did early behavioral genetics studies typically find markers first?
Why does heritability measure variability rather than the average level of a trait, and what does that imply about interpreting a high heritability number?
Explain the finger-count and earring-wearing examples in your own words. What do they show about the relationship between genetic determination and heritability?
How did the Caspi study show a gene-environment interaction for depression, and why does studying a trait in only one environment tend to inflate its apparent heritability?
What did the cross-country study on math and verbal test scores suggest about the role of environment versus genetics in gender-linked cognitive differences?

Vocabulary

gene hunting (noun): The process of searching for the specific gene responsible for a trait.
Gene hunting for diseases began with comparing family DNA.
genetic marker (noun): A stretch of DNA statistically linked to a trait, though not the exact responsible gene.
Early studies found a genetic marker before finding the actual gene.
phenotype (noun): An organism's observable traits, resulting from genes and environment together.
Researchers start from a phenotype difference and search for its genetic basis.
reverse strategy (noun): An approach that starts from a known gene and checks its effects, rather than the other way around.
The reverse strategy began with the vasopressin gene studied in voles.
gene-environment interaction (noun): The way a gene's effect on behavior depends on the surrounding environment.
The depression gene only mattered given childhood stress, showing gene-environment interaction.
quantitative trait loci (noun): Regions of DNA linked to traits that vary by degree, not just presence or absence.
Quantitative trait loci analysis studies many genes affecting a trait at once.
microarray (noun): A lab tool that measures the activity of many genes simultaneously.
Gene microarrays let researchers study thousands of genes at once.
novelty-seeking (noun): A personality trait involving attraction to new and exciting experiences.
A dopamine receptor variant has been linked to novelty-seeking.
variability (noun): The amount of difference or spread found across a group.
Heritability measures the genetic share of variability, not average level.
inflate (verb): To make something appear larger or more important than it really is.
Controlled environments inflate apparent heritability.
counterintuitive (adjective): Different from what common sense would expect.
The finger-count example is a counterintuitive heritability case.
serotonin (noun): A brain chemical linked to mood regulation.
A serotonin-related gene was linked to depression under stress.
antisocial behavior (noun): Actions that harm or disregard the rights and wellbeing of others.
A gene variant was linked to antisocial behavior after childhood abuse.
genotype (noun): The specific genetic makeup of an individual.
Environment often predicts outcomes better than genotype alone.
bioethics (noun): The study of ethical issues arising from biology and medicine.
Testing for adult-onset diseases raises bioethics concerns.
allele (noun): One of two or more versions of the same gene.
Different alleles of a gene can produce different traits.
chromosomal neighborhood (noun): A broad region of a chromosome linked to a trait, before the exact gene is found.
Early gene hunting located a chromosomal neighborhood, not a specific gene.
sequencing (DNA) (noun): The process of reading the exact order of letters in a DNA sample.
DNA sequencing let researchers pin down the exact mutation.
socioeconomic (adjective): Relating to a combination of social and economic factors.
Cross-country studies compared math scores across different socioeconomic conditions.
gender-equality index (noun): A measure of how equal a society treats men and women.
The gap in test scores tracked a country's gender-equality index.

Chapters

0:00 <Untitled Chapter 1>
1:08 Metro Dogs
5:03 Prenatal Environment and Environmental Effects
10:18 Intermediate Physiological Endpoints
10:58 Gene Differences
13:30 Disease Markers
14:36 Bioethics Problem
20:12 Take-Home Message
24:53 Subtle Differences in Facial Expressions
25:34 Bdnf
25:52 Amygdala
28:11 Neurotransmitter Dopamine
28:44 Dopamine
39:51 Heritability
43:16 This Is How Much Water It Retains or the Plant's Iq or Something like that You'Re Measuring some Trait and You'Re Asking Does It Differ as a Function of Which Version of this Gene You Have So You Do Your Study and this Is What You See and You Say Whoa Okay I Went and Looked at this Plant in a Rainforest and We Identify the Genetic Versions of It and Look Very Different the Gene That You Have There Knowing that Gives You a Lot of Predictive Power over What Level of Whatever It Is that You'Re Measuring this Plant Is that's Great so You'Re Going To Get Your Doctorate out of that and You Get some Publications and It's Great and You Finally Stopped Being a Student and It's Terrific
46:01 And What You Wind Up Seeing Here Is if You Can either Know this or the Environment You Want To Know What the Environment Is the Variability Oh Plant Iq Could Come In at 98 a Hundred a Hundred and Two Eight Ten and Twelve the Far More of the Variability in those Six Numbers Is Explained by What Environment It's Going On in Rather than the Genetic Difference That's What Heritability Is Telling You and in the Study like this It Would Tell You that the Heritability Number Is Actually Quite Low because the Amount of Variation due to this Is Far Less Explained by the Gene Type than whether It's the Amazon or the Gobi Desert
55:33 And What Ultimately One Has To Argue Is that It Is Impossible To Ever Say What a Gene Does You Can Only Say What a Gene Does in the Environments Which to Date It Has Been Studied in Okay Let's See that Expanding Even More because You'Ve Got this Ok Let's Jump Ahead Ok so this Is Showing You Now Just How Totally Nutty and Counterintuitive Heritability Terms Actually Are You Ask a Question What's the Heritability of Number of Fingers on Your Hand no Genes Have To Do with the Fact that We Got Five Fingers Instead of Flippers or some Such Thing Genes Have Huge Amounts To Do with It You'Re Not Asking about the Average Number Figure Fingers You'Re Asking about the Variability
1:01:34 This Is What It Would Look like with Zero Percent Heritability There's no Difference at all of the Variation Explained by Gene Variation It's all Environmental So Now We Have a Version That Forces Us To Put in the Same Phrase We Heard about before this Is What Your Data Look like Now and You Now Ask the Question Well What Does Being an Environment What Does Your Environment Have To Do with Your Plant Iq and the Answer Is It Depends on Which Version of the Gene You Have and Now You Ask What Does Having a Certain Version of the Gene Have To Do with Your Plant Iq
1:23:48 Gender Equality Index

From the YouTube description

(April 14, 2010) Robert Sapolsky continues his series addressing the link between behavior and genetics. He covers the complex endeavor of gene isolation and variability and heritability and wrongly eliminated environmental influences in heritability tests -- finding that genes and environment are infinitely interconnected and co-dependent on each other.

Stanford University
http://www.stanford.edu

Stanford Department of Biology
http://biology.stanford.edu/

Stanford University Channel on YouTube
http://www.youtube.com/stanford

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