The only truly DNA-identical people are monozygotic (MZ) twins whose birth is the result of one egg splitting after fertilization. MZ twins inhabit two separate embryos but share 100% of their dna
Dizygotic (DZ) twins are the result of two separate eggs being fertilized by two separate sperm and hence share 50% of their dna
MZ twins are useful for genetic similarity research since they have 100% shared DNA and hence act as each other's controlgroup.
Twin studies start by identifying a proband (the first participant) and the second twin as a comparison.
Studies that use concordance rates (a largescalecomparison between twins in which one variable like depression is measured, concordance occurs when both twins have depression) usually look at specificbehaviors being inherited
Genetic similarities are useful for determining whether there's a biological explanation for behavior or if the environment is also a key contributor to the behavior.
Testing twins rearedapart is extremely difficult to implement though, because not only are twins rare, twins raised separately are even rarer
Twin studies are limited in that factors like common age, sex, and appearance might account for similarities in behavior, not just genetics alone. So it's a bit difficult to disentangleenvironmental and genetic factors.
Kinship or family studies investigate the heritability of a behavior by looking at its incidence over a number of generations. It's usually limited to three generations.