What is the first law of behavioral genetics?

The first law states that all psychological traits are heritable, meaning individual differences in traits like personality, intelligence, and mental health show significant genetic influence. On average, about half of these differences can be attributed to inherited DNA variations.

How is heritability misunderstood?

Heritability is often misunderstood as meaning a trait is solely determined by genes or that it's immutable. Heritability actually describes the proportion of *differences* in a trait within a specific population at a specific time that are due to genetic differences, not the trait in its entirety.

What is the nature of nurture?

The 'nature of nurture' refers to how genetic predispositions influence the environments people choose or are exposed to. For instance, a child's genetic traits can lead them to select friends with similar traits, making correlations between friends appear environmental when they have a genetic component.

Are identical twins raised apart as similar as those raised together?

Studies, such as those involving identical twins raised apart, show they are often remarkably similar in traits, comparable to identical twins raised together. This highlights the significant role of genetics and the lesser role of shared family environment in shaping individuals.

Does epigenetics mean genetics isn't deterministic?

Epigenetics refers to changes in gene expression, which are influenced by the environment. However, what we inherit are DNA sequence differences. While gene expression is crucial, it doesn't invalidate the fundamental role of inherited DNA differences in contributing to traits.

Are complex traits like intelligence or schizophrenia determined by single genes?

No, complex traits and common disorders are not determined by single genes. Instead, they are influenced by thousands of tiny DNA differences, each contributing a small effect. This is known as polygenic influence.

What does it mean for 'the abnormal to be normal' in genetics?

This concept suggests that human traits exist on continuous quantitative dimensions rather than distinct categories of normal or abnormal. Genetic liabilities for conditions like schizophrenia are normally distributed across the population, meaning everyone has some genetic component, and 'disorders' are merely extremes on these spectra.

What are the implications of polygenic scores for diagnosis?

Polygenic scores, which aggregate the effects of thousands of genes, demonstrate that genetic predispositions are quantitative and normally distributed. This challenges traditional diagnostic categories, suggesting they are arbitrary divisions along a continuous spectrum rather than distinct conditions.

Does genetic influence mean parenting doesn't matter?

While genetics plays a significant role, environmental factors, particularly non-shared and idiosyncratic ones, also contribute. Furthermore, parents often respond to their children's genetic predispositions, a concept known as the 'nature of nurture.' Good parenting becomes more about understanding and navigating individual differences.

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The Nature of Human Nature: A Conversation with Robert Plomin (Episode #211)

Sam Harris

Science & Technology4 min read73 min video

Jul 17, 2020|229,795 views|3,528|660

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TL;DR

DNA significantly influences our traits, with half of differences due to genes and the other half to non-systematic environmental factors.

Key Insights

Behavioral genetics has evolved from a taboo subject to a field supported by substantial DNA evidence.

Heritability measures the extent to which individual differences in a trait are due to inherited DNA variations in a specific population.

Approximately 50% of psychological differences between individuals are heritable, with the remaining 50% attributed to environmental factors.

Environmental influences are largely non-shared and idiosyncratic, rather than systematic effects from family or parenting.

The 'nature of nurture' highlights how genes can shape an individual's environment, making correlations between environmental factors and outcomes often genetic in disguise.

Complex traits and disorders are influenced by thousands of genes, each contributing small effects, rather than single genes.

Psychological traits exist on quantitative dimensions, and diagnoses are often arbitrary divisions of these continuous spectra.

THE EVOLUTION OF BEHAVIORAL GENETICS AND ITS TABOOS

The discussion opens by framing behavioral genetics as a field that has grappled with significant controversy and taboos, particularly concerning the study of genetic influences on human psychology. Early in its development, the field was dominated by environmentalist perspectives, making it professionally risky to explore genetic contributions. However, decades of twin, adoption, and now DNA studies have accumulated substantial evidence, convincing many scientists that genetic factors play a significant role in almost all psychological traits, explaining a considerable portion of individual differences.

UNDERSTANDING HERITABILITY: MORE THAN JUST GENES

Heritability, a core concept in behavioral genetics, is often misunderstood. It is a descriptive statistic that quantifies the extent to which observed differences in a trait within a population are attributable to inherited DNA differences. It's crucial to distinguish heritability from determinism; a highly heritable trait can still be heavily influenced by environmental factors, and the heritability estimate applies to variations within a specific population at a given time, not to what could be or to individual destiny.

THE FIRST LAW AND THE MYSTERY OF NON-SHARED ENVIRONMENT

The 'first law of behavioral genetics' states that all psychological traits are heritable, meaning individual differences show significant genetic influence. While genes account for roughly 50% of these differences, the remaining variance is environmental. Paradoxically, studies show that this 'nurture' is not primarily derived from shared family environments (like parenting styles or socioeconomic status), but rather from 'non-shared environment'—idiosyncratic experiences that make siblings different, the precise sources of which remain largely elusive and seem more random than systematic.

THE NATURE OF NURTURE: GENES SHAPING ENVIRONMENT

A significant nuance is the 'nature of nurture,' where genes not only influence our traits directly but also shape the environments we encounter and create. Correlations observed between environmental factors (like parental reading to children) and outcomes are often a reflection of genetic predispositions. Individuals with certain genetic tendencies are more likely to select or evoke environments that align with those tendencies, meaning many apparent environmental influences are actually genetic effects in disguise.

METHODOLOGIES: TWINS, ADOPTIONS, AND DNA'S REVOLUTION

The compelling evidence for genetic influence comes from rigorous methodologies like twin studies (comparing identical vs. fraternal twins) and adoption studies (comparing adopted children to their biological and adoptive parents). These methods allow researchers to disentangle genetic and environmental influences. The advent of DNA technology has revolutionized the field, moving beyond correlational studies to directly examine genetic variations and their predictive power for complex traits, though it reveals that most traits are influenced by thousands of genes.

THE POLYGENIC NATURE OF COMPLEX TRAITS AND DISORDERS

Complex human traits and common disorders are not governed by single genes but by the combined action of thousands of genes, each contributing a small effect. This polygenic architecture means that genetic liability for traits like schizophrenia or intelligence exists on a quantitative spectrum. The concept of discrete disorders is challenged, as most traits, including susceptibility to illness, are seen as normally distributed dimensions where 'abnormal' is merely a point on a continuous range influenced by additive genetic factors that are present in varying degrees across the population.

IMPLICATIONS FOR PARENTING, SOCIAL POLICY, AND DIAGNOSIS

Understanding the genetic underpinnings of human behavior has profound implications for parenting, social policy, and our understanding of psychological disorders. While genetic influences are substantial, the non-random nature of parenting and societal structures still plays a role. Furthermore, the realization that traits are dimensional challenges traditional diagnostic frameworks in psychiatry and psychology, suggesting a shift towards understanding individuals on continuous spectra rather than within categorical boxes. The idea of 'curing' a disorder is reframed as managing or alleviating symptoms along a quantitative dimension.

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