Revise Genetics and behaviour for IB Psychology (1.3)

DNA is organised into chromosomes inside the nucleus of our cells. A gene is a segment of DNA that codes for a protein or function; your complete set of genes is your genotype. What you can actually observe — traits, personality, vulnerability to disorders — is your phenotype, and the phenotype is produced by the genotype interacting with the environment across your whole life. The single most important idea in this topic follows from that: genes influence behaviour by shaping probabilities and tendencies, not by fixing outcomes. A genetic predisposition raises the likelihood that a behaviour appears; whether it actually does often depends on experience. Treating a gene as a direct cause of a behaviour is called genetic determinism, and the research below repeatedly shows why that view is too simple.

We cannot ethically manipulate human genes, so psychologists use natural variation in genetic relatedness to infer genetic influence. Each method is a way of holding either genes or environment roughly constant so the other can be examined. None is a true experiment, which is why every one of them supports correlational, not causal, conclusions.

Because concordance rates in these studies are almost never 100% — even for MZ twins with essentially identical DNA — the studies themselves demonstrate that the environment must contribute. That built-in gap is one of the most quotable evaluation points in the whole topic.

Heritability estimates are everywhere in this topic, and they are constantly misread. Heritability is a population statistic: it estimates the proportion of the VARIATION in a trait across a group that is associated with genetic differences among its members. It does not describe any single person — you cannot say '70% of this child's intelligence is genetic'. It is also specific to the population and environment measured: change the environment (say, remove severe deprivation) and the heritability figure itself can change. And a high heritability does not make the environment powerless: a trait can be highly heritable yet still be strongly shifted, on average, by environmental change.

The modern picture replaces 'genes versus environment' with gene-environment interaction (GxE): the effect of a gene depends on the environment, and the effect of the environment depends on the genotype. The diathesis-stress model is the classic case — a genetic vulnerability (diathesis) is expressed only when environmental stress is present. Epigenetics gives this a biological mechanism. It studies changes in gene EXPRESSION — genes switched on or off, for instance through DNA methylation — that are triggered by the environment without altering the DNA sequence itself. In other words, experience can change which genes are active. Epigenetics is examined here at overview level: you need the idea that environment can regulate gene expression, and why that is further evidence against simple genetic determinism.

Evolutionary psychology argues that behaviours which improved survival and reproduction in ancestral environments were naturally selected and so remain common today. Disgust, for example, can be explained as an evolved response that reduced contact with sources of disease. The approach is genuinely useful for generating hypotheses about universal human tendencies. But it draws heavy criticism. Many evolutionary explanations are post-hoc 'just-so stories' constructed after the fact and hard to falsify, because we cannot rerun ancestral conditions. The approach can also underplay culture and learning, and risks the naturalistic fallacy — treating 'evolved' as 'natural' and therefore acceptable. Use it as one lens among several, and always pair it with these limitations.

Genetics is one pillar of the biological approach; it sits alongside the brain, hormones and neurotransmitters covered elsewhere in Unit 1. The evaluative moves you have practised here — predisposition not determinism, correlation not causation, heritability as a population statistic — transfer directly to those topics and to any Paper 1 question that asks you to weigh a biological explanation of behaviour.