# Genetic inheritance

> IB Psychology · IB Psychology 2027 First Assessment
> Source: https://www.owlsprep.com/study/ib-psychology-bio-genetic-inheritance/

The extent to which human behaviour may be inherited.

## Learning objectives

- The extent to which human behaviour may be inherited.

## How often is this tested?

Based on OwlsPrep's analysis of official CIE IB_PSYCH P1 past papers (2019–2025): **Genetic inheritance** appears **17 times** in the last 7 years — **7.5%** of all P1 questions (17 of 228).

Most-tested forms: Essay: Discuss (22 marks) (7), SAQ: Describe (9 marks) (6), SAQ: Outline (9 marks) (2), Essay: Evaluate (22 marks) (1), Essay (22 marks) (1).

## Core concept

This concept asks how far behaviour is inherited. Researchers estimate heritability using twin studies (comparing identical vs fraternal twins), plus family and adoption studies. A heritability figure — say ~37% for depression — is a population estimate of how much of the variation is linked to genes, not a claim that an individual is 37% 'destined'. Genes set predispositions that then interact with the environment, and can be switched on and off by it (epigenetics).

## Key studies

IB Psychology answers must be supported by named studies. Learn these to cite as evidence:

**Sullivan, Neale & Kendler (2000)** — Twin meta-analysis put the heritability of depression at about 37%. — *Quantifies the genetic contribution — and, by leaving ~63%, the environmental one.*

**Kendler et al. (2006)** — In Swedish twins, heritability of major depression was higher in women (42%) than men (29%). — *Shows heritability is not a fixed constant — it varies by group.*

## Evaluation (AO3)

**Strength — separates genes from environment** — Twin and adoption designs let researchers estimate the genetic share of variation that ordinary family studies confound.

**Limitation — the equal-environments assumption** — Twin studies assume identical and fraternal twins share environments equally, which may inflate heritability; the figure is population-specific, not individual.

## Scope

> **note**
>
> Genetic inheritance of behaviour is the focus, including epigenetics (which the 2027 guide places in the biological approach). Its developmental elaboration — how early experience switches genes on/off — is detailed in Human development (Weaver/McGowan).

## Common pitfalls

- **Wrong:** 'Heritability of 37%' means 37% of my depression is genetic.
  - Why it fails: A frequently penalised misconception in IB Psychology exams.
  - Correct: Heritability describes variation across a population, not a proportion within one person, and it says nothing about how changeable the trait is.

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-psychology-bio-genetic-inheritance/