Genes and environment work together to shape who you are. It's not nature or nurture; both play crucial roles, and they influence each other in ways that shift across your lifespan. This section covers the main models for how genes and environments interact, how we measure genetic influence, and why the balance between shared family experiences and unique individual experiences changes as you grow.
Genetic and Environmental Influences
Nature vs. Nurture
Nature refers to genetic influences on development, while nurture encompasses environmental influences. For decades, psychologists debated which one mattered more, but that framing turns out to be misleading. Modern developmental psychology recognizes that genes and environment aren't independent forces pulling in opposite directions. They have bidirectional influences on each other, a concept called gene-environment interplay. Your genes shape the environments you encounter, and your environments affect how your genes are expressed.
Gene-Environment Interplay
There are two broad categories here, and the distinction matters: gene-environment correlation and gene-environment interaction. They sound similar but describe different processes.
Gene-environment correlation (rGE) occurs when a person's genotype is statistically associated with the environments they experience. There are three types:
- Passive rGE: Parents provide both genes and environment. A musically talented parent passes on genes linked to musical ability and also fills the home with instruments and music lessons. The child didn't choose that environment; it came packaged with the genes.
- Evocative rGE: A person's genetically influenced traits pull certain responses from other people. A naturally friendly toddler smiles at strangers and gets more positive social interaction in return, which further reinforces social skills.
- Active rGE: Individuals seek out environments that fit their genetic predispositions. A child with high cognitive ability gravitates toward books, puzzles, or academically challenging friends. This type becomes more prominent as children gain autonomy with age.
Gene-environment interaction (GxE) is different. Here, the effect of an environment on a trait depends on a person's genotype. Two major models describe this:
- Diathesis-stress model: Some individuals carry a genetic vulnerability (the diathesis) that only leads to problems when activated by stressful environmental conditions. Under low stress, they develop normally; under high stress, they're at elevated risk.
- Differential susceptibility model: This extends the diathesis-stress idea. Certain genotypes aren't just more vulnerable to bad environments; they're more responsive to all environments, good and bad. These individuals are sometimes called orchid children because they wilt in harsh conditions but flourish in supportive ones. Dandelion children, by contrast, develop about the same regardless of environmental quality.
Heritability
Heritability is a population-level statistic that estimates the proportion of variation in a trait attributable to genetic differences between individuals. It ranges from 0 (none of the variation is genetic) to 1.0 (all of the variation is genetic).
A few things students commonly get wrong about heritability:
- It describes a population, not an individual. You can't say "80% of your intelligence is genetic." You can say "in this population, about 80% of the variation in intelligence scores is associated with genetic differences."
- It's specific to a particular population in a particular environment. Change the environment and the heritability estimate can change too. In a society where everyone gets identical nutrition, heritability of height would be very high because the remaining differences would mostly be genetic. In a society with huge nutritional inequality, environment would account for more variation and heritability would drop.
- High heritability does not mean a trait is fixed. Height is highly heritable, yet average height has increased dramatically over the past century due to improved nutrition.
Developmental Plasticity
Reaction Norms
A reaction range is the range of possible phenotypes a single genotype can produce across different environments. The graphical representation of this is called a norm of reaction: it plots the phenotype a given genotype produces at each level of an environmental variable.
Different genotypes can show very different norms of reaction. One genotype might produce similar outcomes across a wide range of environments (a flat line), while another might be highly sensitive to environmental changes (a steep slope). Broader reaction norms indicate higher developmental plasticity, meaning the trait is more malleable. Narrower reaction norms suggest the trait is more resistant to environmental influence.
Canalization
Canalization is the buffering of a phenotype against environmental or genetic disruptions. A highly canalized trait develops the same way across a wide range of conditions. Think of it like a ball rolling down a deep canal: small bumps won't knock it off course.
- Many species-typical traits (like developing two eyes or learning to walk) are highly canalized because their reliable development was critical for survival.
- Decanalization can occur in novel or extreme environments, causing previously hidden genetic variation to surface. Under normal conditions, that variation was masked by canalization; under stress, it becomes visible in the phenotype.
Environmental Factors
Shared vs. Non-Shared Environment
Shared environment includes factors that make siblings raised in the same family more similar to each other: the same household, same socioeconomic status, same neighborhood. Non-shared environment encompasses influences unique to each sibling: different friend groups, different teachers, or even differential treatment from the same parents.
One of the more surprising findings in behavioral genetics is that, for many psychological traits, non-shared environment accounts for more variation than shared environment. This is a big part of why identical twins raised together, despite sharing 100% of their genes and the same household, are not psychologically identical. Their unique experiences pull them apart.
Environmental Influences Across Development
The relative weight of shared and non-shared environment shifts over time:
- In early childhood, shared environment tends to be more influential because family life dominates a young child's world.
- As children age, they spend more time outside the home with peers, at school, and eventually at work. These non-shared experiences become increasingly important in shaping individual differences.
Gene-environment correlations also shift with development. Young children mostly experience passive rGE (environments chosen by parents). As they gain autonomy, active rGE takes over and they increasingly select their own environments. This is one reason heritability estimates for traits like intelligence tend to increase with age: people are progressively curating environments that match and amplify their genetic tendencies.
Interventions aimed at enhancing development may be most effective in early childhood, when developmental plasticity is greatest and shared environmental influences are strongest. That's the period when changing the family environment can have the broadest impact.