Twin Studies (Genetic Similarity)

Define genetic similarity.

The degree to which individuals share the same genes.

How genetically similar are humans to each other?

Humans are approximately 99.9% genetically identical.

What causes variations in physical, behavioural, and personality traits among humans?

Small genetic differences in DNA.

Define monozygotic (MZ) twins.

Twins that come from a single fertilised egg that splits in two.

How genetically identical are monozygotic twins?

They share 100% of their DNA.

Define dizygotic (DZ) twins.

Twins that come from two separate eggs fertilised by two separate sperm.

How genetically identical are dizygotic twins?

They share about 50% of their DNA.

Why are monozygotic twins valuable for genetic research?

They provide natural control groups for studying biological influences on behaviour.

What do twin studies compare to assess genetic influence?

Concordance rates — the likelihood that both twins share a trait or disorder.

What does a higher concordance rate in MZ twins compared to DZ twins suggest?

A stronger genetic influence on the trait or disorder.

(+) One advantage of twin studies regarding mental disorders.

They provide strong evidence for biological roots of certain disorders.

(+) How have twin study findings benefited psychology?

They help identify genetic vulnerability and inform prevention efforts.

(+) What supports reliability of twin study results across cultures?

High cross-cultural consistency of concordance rates.

(-) Main limitation of twin studies regarding environment.

MZ twins are seldom raised apart, so shared environments may influence results.

(-) Why might DZ twins experience more environmental differences than MZ twins?

Due to differences in appearance, temperament, or social treatment.