How do genes and environment influence behaviour?

For: Readers evaluating claims that behavioural differences are inherited or produced by upbringing.

A child may resemble a parent in talkativeness, reading or problem solving. That resemblance does not, by itself, explain how it arose. Three questions need separating: how an ability develops, why people differ on a measure, and what would happen if their circumstances changed. A genetic explanation of differences does not automatically answer the other two.

First specify the behaviour and how it was measured. A questionnaire rating, a test score and an observed action are different kinds of evidence. The MIT intelligence and personality sessions are useful for this preparatory step. An explanation of differences in one measure should not silently become an explanation of the whole person.

Families share experiences as well as genetic relationships. Noba’s nature–nurture chapter explains why adoption and twin comparisons help researchers separate possible influences. Adoption separates biological parentage from being raised by those parents, but placement is not a random experiment. Identical and non-identical twin comparisons provide different degrees of genetic relatedness; interpreting their resemblance also requires attention to their environments. These designs are informative comparisons, not automatic proof that every observed family resemblance is genetic.

Heritability concerns variation across people in a specified population and set of circumstances. In a hypothetical study, an estimate of 0.6 would attribute 60% of the measured variation to genetic variation under the study’s model. It would not mean that 60% of each person’s behaviour comes from genes. Nor would it identify the responsible genes, explain the developmental process, or tell us how readily the trait could change. The MedlinePlus explanation makes these boundaries explicit. The estimate can change when the population or circumstances change.

Variation within groups and differences between their averages are also separate questions. A high within-group heritability estimate cannot by itself establish why two groups have different averages. The optional 2007 Yale lecture’s Chapter 5 illustrates that logical distinction. Use it as a historical methods comparison: its later sweeping conclusions about parenting are not conclusions of this guide.

Environment includes physical and social conditions, not merely parenting. Genes and environments can also interact. In the narrower research sense, this means that the effect of an exposure differs with genetic background, or the effect of genetic differences depends on the exposure. The NHGRI glossary explains this using disease risk. It supplies a definition, not evidence that a particular behavioural interaction exists. Saying that both genes and experience matter does not demonstrate an interaction; a specific claim needs a specified outcome, comparison and evidence.

The starting MIT lecture asks a different, complementary question: how does organised brain function develop? Its opening discussion of face perception considers whether a young animal’s response reflects faces specifically, movement more generally, or experience already acquired. This is an example of turning a nature–nurture slogan into competing explanations and a better comparison. It is not a heritability calculation. The continuation examines how connectivity might constrain development, while explaining limitations of the imaging method.

Finally, evidence about existing differences is not a direct test of an intervention. Noba’s example of parents reading to children shows the problem: an association with later reading scores leaves several possible pathways open. A suitable intervention study addresses a different question. High heritability alone neither establishes that change is impossible nor shows that a proposed programme works. The exercise below practises keeping those conclusions separate.

Next step

Return to the intelligence or personality session and identify the exact measure and comparison behind one claim. Separate the population-difference question from the evidence needed to establish a developmental mechanism or an effective intervention.

Limits of this route

The selected teaching dates from Yale 2007, MIT psychology 2011, and the MIT 2019 brain course, whose continuation is explicitly labelled 2018. The two brain transcript openings were read, not the full recordings; the psychology selections were checked against session pages, and Yale against the specified transcript sections. MedlinePlus’s page is dated September 2021; the NHGRI definition was retrieved for this check. Noba is used for study-design reasoning, not as a current review of genetic discovery, candidate-gene effects or epigenetic inheritance. Yale’s broad parenting claims and MIT’s birth-order claims are not adopted. No claim is made that all family-design assumptions are satisfied in any particular study. Genome-wide association, polygenic prediction, clinical testing and personal genetic advice are outside scope. Agent source inspection is not full-video viewing, human peer review or independent scientific validation.