Modern WisdomHow Do Genes Influence Our Behaviour? - Robert Plomin | Modern Wisdom Podcast 353
CHAPTERS
- 0:00 – 1:40
Adopted siblings and the shock: shared family environment often doesn’t make kids similar
The conversation opens with a striking adoption-study result: adopted siblings raised together show near-zero correlation in IQ. This sets up the core ‘uncomfortable’ theme of Plomin’s work—genes explain more about differences between people than most expect, while shared family effects are smaller than assumed.
- •Adoptive siblings share home/parenting but not genes; their IQ correlation is ~0
- •Biological siblings correlate modestly in IQ, increasing with age
- •The finding challenges common assumptions about parenting and home environment
- •Frames the episode’s focus: individual differences and what really drives them
- 1:40 – 6:43
Defining behavioral genetics: DNA differences that explain psychological differences
Plomin defines behavioral genetics as the study of how inherited DNA differences relate to differences in behavior (psychology), not human universals. He clarifies that while humans share ~99.9% of DNA, the small portion that differs still contains millions of variants that can shape susceptibility and traits.
- •Behavioral genetics parallels medical genetics but targets behavior/psychology
- •Focus is on individual differences (e.g., depression, schizophrenia, personality, cognition)
- •Humans are mostly genetically identical; the key is the variable fraction
- •Contrasts universals (evolutionary psychology) with variance between individuals
- 6:43 – 12:07
Nature vs nurture, heritability, and three big misconceptions (height/weight examples)
Plomin introduces heritability as a population statistic describing variance between people, not how much of ‘you’ is genetic. Using height (~90% heritable) and weight/BMI (~70%), he dismantles common errors: heritability isn’t personal causation, genetics isn’t destiny, and ‘what is’ differs from ‘what could be’ with interventions.
- •Heritability refers to differences in a population at a given time/place
- •Height is extremely heritable; weight/BMI is surprisingly highly heritable
- •Misconception #1: not ‘90% of my height came from genes’—it’s variance between people
- •Misconception #2: genetic influence is usually probabilistic, not deterministic
- •Misconception #3: current population patterns don’t define what an individual can change
- 12:07 – 17:53
Polygenic risk, appetite and food cues: how genes express through behavior
The discussion moves from single-gene determinism to polygenic propensities and how they show up in daily life. Plomin explains that BMI can already be predicted partly via polygenic scores, and that genetic risk often manifests through psychological mechanisms like satiety and cue-responsiveness, with implications for stigma and self-management.
- •Most common traits/disorders are influenced by thousands of tiny genetic effects
- •Polygenic scores can predict a meaningful slice of BMI differences (~10% with DNA alone)
- •Genetic risk can act via satiety, cravings, habits, and responsiveness to cues
- •Different pathways can lead to the same outcome (e.g., ‘not everyone gets fat the same way’)
- •Implication: more compassion/less moralizing (e.g., around weight)
- 17:53 – 22:39
How scientists separate genes and environment: twins, adoption, and measurement basics
Plomin explains how behavioral genetics is done without needing to ‘see genes’ directly, using classic twin and adoption designs. He also addresses measurement concerns in psychology, arguing that despite reliance on questionnaires for some traits, many measures show meaningful stability and reliability over time.
- •Twin method: identical (100% shared DNA) vs fraternal (≈50%) similarity
- •Adoption method: disentangles shared home environment from genetic relatedness
- •Reliability and stability matter; cognitive tests are especially robust measures
- •Across psychological traits, twin/adoption evidence converges on substantial heritability
- 22:39 – 27:42
The massive UK twin cohort and cross-country replication (and SES interaction debates)
Plomin describes building a landmark England/Wales cohort of ~10,000 twin pairs followed repeatedly from early childhood into adulthood. He notes that millions of twin pairs worldwide show broadly similar heritability estimates, and discusses contested claims that heritability drops in highly deprived contexts (mostly an American debate).
- •UK cohort: ~10,000 twin pairs recruited from birth records; assessed 14 times to age 25
- •High participation rates and careful ethics around bereavement/illness exclusions
- •Worldwide evidence: ~3 million twin pairs across ~3,000 reports with similar patterns
- •Gene–environment interaction claim: possibly lower heritability in low-SES US samples, not consistently replicated
- 27:42 – 32:59
From ‘genes for X’ to polygenicity: why complex traits resist simple biology stories
Chris probes whether there are ‘genes for’ specific behaviors; Plomin emphasizes that complex traits are massively polygenic, undermining simplistic one-gene explanations. He argues that understanding causes doesn’t automatically yield cures, and critiques the medicalized hunt for single brain ‘lesions’ behind learning/behavior problems.
- •Single-gene disorders exist, but common traits/disorders are polygenic
- •Polygenicity makes simple ‘this gene/this brain area causes that behavior’ stories unlikely
- •Key idea: cures aren’t necessarily related to causes for complex outcomes
- •Critique of over-medicalization (e.g., dyslexia/ADHD framed as discrete diseases)
- 32:59 – 36:32
“No disorders, only quantitative dimensions”: normal distributions and diagnosis skepticism
Plomin and Chris argue that many ‘disorders’ reflect extremes on continuous dimensions rather than distinct categories. This viewpoint reduces ‘us vs them’ thinking (e.g., schizophrenia) and shifts goals from ‘curing’ labels to reducing symptoms and improving functioning on a spectrum.
- •Genetic risk is continuously distributed; categories are often thresholds on a curve
- •Diagnoses can create false dichotomies (‘you have it’ vs ‘you don’t’)
- •Most people carry some risk alleles; outcomes remain probabilistic
- •Practical shift: target symptom reduction and support rather than categorical cures
- 36:32 – 49:27
Heritability tour: aggression, IQ across the lifespan, schooling, and why IQ heritability rises
Plomin gives approximate heritability figures for various traits and explains why IQ heritability increases with age. He highlights gene–environment correlation: people increasingly select and shape environments aligned with their genetic propensities, making differences ‘snowball’ over time.
- •Personality traits often ~40% heritable; some early-life callous-unemotional traits are higher
- •IQ heritability rises with age (roughly 20% infancy → 60% young adulthood, even higher later)
- •Mechanism: gene–environment correlation (selection, modification, creation of environments)
- •Educational achievement can be highly heritable early and stays ~stable while IQ rises
- •Policy/teaching implication: kids differ in learning appetites and trajectories
- 49:27 – 55:58
Happiness and wellbeing: genetic influence plus philosophy, meditation, and stoic reframing
The conversation shifts to wellbeing, with Plomin citing moderate heritability for happiness/wellbeing measures. They broaden into practical philosophy—meditation, non-duality, and stoicism—arguing that chasing happiness as an outcome can backfire, while presence, peace, and reframing reduce suffering.
- •Wellbeing/happiness measures often show ~40% heritability
- •Distinguishing happiness-as-motion from peace-as-stillness
- •Stoic concepts: hedonic adaptation, negative visualization, wanting what you have
- •Meditation as a tool to relate differently to thoughts and sensations
- •Philosophy as ‘practical psychology’ when tied to lived experience
- 55:58 – 58:42
Sexual preference and relationships: genetic influence without ‘a gay gene’ (and divorce heritability)
Plomin addresses sexual preference, rejecting the simplistic ‘single gay gene’ narrative while affirming genetic influence at the population level. He also notes that relationship outcomes like divorce show some heritability, likely via underlying traits (novelty-seeking, temperament) rather than genes ‘for divorce.’
- •No single ‘gay gene’; sexual preference shows genetic influence in twin research
- •Genetics can support the idea that orientation isn’t merely a lifestyle choice
- •Divorce shows modest heritability (roughly ~30% as discussed)
- •Relationship outcomes reflect deeper traits rather than single direct genetic causes
- 58:42 – 1:03:32
Why kids aren’t just blends of parents: genetic lottery within families, regression to the mean, and ‘genetic castes’ fears
Plomin explains why children can differ dramatically from their parents and siblings: genes are discrete units and polygenic scores vary widely within families. He introduces regression to the mean (exceptional parents tend to have less-exceptional children on average) and argues this variability counters simplistic fears of permanent genetic castes—assuming mobility remains possible.
- •Mendelian inheritance means genes don’t simply ‘blend’; siblings differ substantially
- •Polygenic scores are normally distributed even within the same family
- •Regression to the mean: extreme parental traits tend to move toward population average in offspring
- •Concerns about ‘genetic caste systems’ depend on mobility and assortative mating
- •Anecdote: Plomin vs his sister illustrates within-family divergence
- 1:03:32 – 1:09:45
The ‘other half’: non-shared environment, the “gloomy prospect,” and chance turning points
Plomin argues that the environmental component of individual differences is largely non-shared—experiences that make siblings different rather than similar. After decades of searching, researchers struggle to find stable, measurable ‘systematic’ environmental causes, leading to the ‘gloomy prospect’ that chance events and idiosyncratic experiences play a major role.
- •Shared family environment often contributes less than expected to many outcomes
- •Non-shared environment: peers, accidents, illnesses, unique experiences
- •Difficulty identifying replicable environmental variables despite extensive research
- •‘Gloomy prospect’: stochastic, one-off turning points may matter a lot
- •Links to broader ideas about unpredictability and narrative ‘sliding doors’ moments
- 1:09:45 – 1:23:52
Parenting implications: “Parents matter, but they don’t make a difference” + schools and ‘added value’
Plomin clarifies the provocative idea that parenting matters for care and relationship, but typically has limited impact on long-term trait differences compared to genetics and non-shared environment. He also discusses school effects: while parents chase Ofsted ratings and private schools, measured school ‘added value’ on exam outcomes appears small once selection and SES are accounted for.
- •Parenting books often overclaim control and underuse evidence
- •Best role: support a child’s aptitudes/appetites; provide opportunities, not rigid scripts
- •Trying to ‘systematize chance’ is costly and usually ineffective; “go with the genetic flow”
- •School-quality effects on GCSE variance are small (and smaller after SES correction)
- •Private schools may reflect selection rather than strong causal improvement in attainment
- 1:23:52 – 1:42:07
DNA dating, assortative mating, and equality of opportunity: living with genetic knowledge
They explore emerging DNA-based matchmaking and genetic screening—especially for rare recessive single-gene disorders—while warning that current ‘DNA dating’ marketing overpromises on personality prediction. The episode closes on social implications: heritability can rise as environments equalize, challenging equality-of-outcome ideals, and ends with a personal takeaway about acceptance and steering lightly with life’s currents.
- •DNA dating exists; personality prediction is still weak, but screening for recessive diseases is powerful
- •Whole-genome sequencing could identify shared carrier mutations and inform IVF decisions
- •Assortative mating is strongest for cognitive ability (partners correlate highly on IQ-related traits)
- •Equality of opportunity can increase heritability of outcomes; equality of outcome remains unrealistic
- •Closing philosophy: listen to ‘genetic whispers,’ release overcontrol, and work with constraints
