Genes and environment work together to shape who we are. It's not just nature or nurture—both play crucial roles. This interplay is complex, with genes influencing our environments and environments affecting how our genes work.

Heritability measures genetic influence on traits in populations, but it's not set in stone. Our ability to adapt to different environments (plasticity) varies. As we grow, the balance of shared family experiences and unique individual experiences shifts, impacting our development.

Genetic and Environmental Influences

Nature vs. Nurture

• Nature refers to genetic influences on development while nurture encompasses environmental influences
• The nature vs. nurture debate historically argued whether genes or environment played a larger role in shaping development
• Modern perspectives recognize that both nature and nurture interact in complex ways to influence development
• Genes and environment are not independent factors but have bidirectional influences on each other (gene-environment interplay)

Gene-Environment Interplay

• Gene-environment correlation occurs when an individual's genotype is associated with their environmental exposure
  • Passive gene-environment correlation happens when parents provide both genes and environment (musical parents passing on musical genes and providing music lessons)
  • Evocative gene-environment correlation occurs when an individual's genetically influenced traits evoke certain responses from the environment (a friendly child receiving more positive social interactions)
  • Active gene-environment correlation involves individuals seeking out environments that match their genetic predispositions (an intelligent child seeking mentally stimulating activities)
• Gene-environment interaction occurs when environmental influences on a trait differ depending on an individual's genotype
  • Diathesis-stress model proposes that some individuals have a genetic vulnerability (diathesis) that is activated under stressful environmental conditions
  • Differential susceptibility model suggests some genotypes are more susceptible to both positive and negative environmental influences (orchid children) while others are less affected by environment (dandelion children)

Heritability

• Heritability is a population statistic that estimates the proportion of variation in a trait that is due to genetic differences between individuals
• Heritability ranges from 0 (no genetic influence) to 1 (entirely genetically determined)
• Heritability estimates are specific to a particular population in a particular environment and can change if environment changes
• High heritability does not mean a trait cannot be changed by the environment as heritability is not an absolute measure of genetic influence

Developmental Plasticity

Reaction Norms

• Reaction range refers to the range of possible phenotypes that a single genotype can produce in different environments
• Norms of reaction depict the relationship between a genotype and the phenotypes it produces across a range of environments
  • Different genotypes can show different patterns of responses to environmental variation in their norms of reaction
• Broader reaction norms indicate higher developmental plasticity while narrower reaction norms suggest less malleability

Canalization

• Canalization is the reduced sensitivity of a phenotype to environmental or genetic perturbations
• Highly canalized traits show little phenotypic variation in response to environmental changes or mutations
• Canalization evolved to stabilize the development of important traits against environmental and genetic variability
• Decanalization can occur in novel or stressful environments, leading to the expression of previously hidden genetic variation

Environmental Factors

Shared vs. Non-Shared Environment

• Shared environment includes environmental factors that make siblings from the same family more similar to each other (growing up in the same home, having the same socioeconomic status)
• Non-shared environment encompasses environmental influences that are unique to each sibling and make them different from one another (different peer groups, differential parental treatment)
• Studies comparing similarity between identical and fraternal twins suggest non-shared environment accounts for more variation in many psychological traits than shared environment
• The non-shared environment helps explain why genetically identical individuals (identical twins) are not exactly alike

Environmental Influences Across Development

• The relative importance of shared and non-shared environmental factors can shift over the course of development
  • Shared environment tends to be more influential in early childhood when family environment is central
  • As individuals age, unique non-shared experiences outside the family (school, peers, work) become increasingly important
• Gene-environment correlations and interactions can also change with development as individuals gain more autonomy to shape their environments
• Interventions to enhance positive development may be most effective in early childhood when developmental plasticity is greatest and shared environmental influences are strongest