Twin study

A research design that compares identical and fraternal twins to estimate how genetic and environmental differences contribute to variation in a trait.

Monozygotic twins: Twins who develop from one fertilized egg that splits, sharing nearly all inherited genetic variation. Their close genetic similarity supplies the design’s higher-genetic-relatedness comparison.
Adoption study: A research design that compares adopted people with biological and adoptive relatives to study genetic and environmental influences. It separates family resemblance through biological relatedness from resemblance through rearing in a different way.
Minnesota Study of Twins Reared Apart: A longitudinal research project examining twins and siblings separated early in life and raised apart. Its separated pairs offered evidence about resemblance when twins did not share a household.
Nature versus nurture: A broad debate about how inherited biology and environmental experience shape traits and behavior. Twin-study estimates gave this debate quantitative evidence while showing that both sources can matter.
Dizygotic twins: Twins who develop from two separate fertilized eggs and share, on average, half their segregating genetic variation. Their average genetic relatedness provides the comparison with identical twins.
Family study: A study of trait resemblance among relatives, whose genetic similarity and shared environments vary across family relationships. Unlike twin studies, family studies usually cannot isolate genetic relatedness from shared upbringing as sharply.
Swedish Twin Registry: A population-based registry of Swedish twins used for epidemiological and behavioral research. It enables large-scale twin studies across diseases, behaviors, and life stages.
Gene–environment correlation: The process by which genetic differences are associated with differences in environmental exposure or experience. It complicates interpretations when genetically influenced traits also shape the environments twins encounter.
Intraclass correlation: A statistic measuring similarity among observations grouped within the same unit, such as pairs of twins. Twin resemblance is often summarized with correlations within identical and fraternal pairs.
Genome-wide association study: A study testing genetic variants across many genomes for statistical association with a trait. It links measured variants to traits, whereas twin studies infer genetic influence from resemblance patterns.

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