behavioural genetics Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" ...

, DeFries–Fulker (DF) regression, also sometimes called DeFries–Fulker extremes analysis, is a type of multiple regression analysis designed for estimating the magnitude of genetic and environmental effects in

twin studies Twin studies are studies conducted on Identical twin, identical or Fraternal twin, fraternal twins. They aim to reveal the importance of environmental and genetics, genetic influences for traits, phenotypes, and disorders. Twin research is consid ...

. It is named after John C. DeFries and

David Fulker David William Fulker (8 March 1937 – 9 July 1998) was a British behavioural geneticist at the University of Colorado's Institute for Behavioral Genetics. Among positions of esteem, he was elected president of the Behavior Genetics Associatio ...

, who first proposed it in 1985. It was originally developed to assess

heritability Heritability is a statistic used in the fields of Animal husbandry, breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. T ...

reading disability A reading disability is a condition in which a person displays difficulty reading. Examples of reading disabilities include developmental dyslexia and alexia (acquired dyslexia). Definition The National Institute of Neurological Disorders and ...

in twin studies, but it has since been used to assess the heritability of other cognitive traits, and has also been applied to non-twin methodologies.

Approach

DeFries–Fulker regression analysis is based on the differences in the magnitude of

regression to the mean Regression or regressions may refer to: Arts and entertainment * ''Regression'' (film), a 2015 horror film by Alejandro Amenábar, starring Ethan Hawke and Emma Watson * ''Regression'' (magazine), an Australian punk rock fanzine (1982–1984) * ...

in a genetic trait between

monozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of Twin Last Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two e ...

(MZ) and

dizygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of Twin Last Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two e ...

(DZ) twins. In DF regression, the first step is to select

proband In medical genetics and other medical fields, a proband, propositus (male proband), or proposita (female proband) is a particular subject (human or other animal) being studied or reported on. On pedigrees, the proband is noted with a square (male) ...

s in a twin study with extreme scores on the trait being studied, and to regression to the mean among their co-twins depending on whether the probands are part of a MZ or DZ twin pair. DF regression is thus based on the assumption that, to the extent that genetic factors play a role in causing the extreme variation in the studied trait, the magnitude of regression to the mean should be greater in DZ co-twins of probands than in their MZ counterparts. This should result in MZ co-twins being more similar to their probands on the trait under study than DZ twins are to theirs.

Modifications

Since the DF regression approach was first proposed in 1985, other researchers have proposed modified versions of the DF model that take additional factors into account, including observed covariates and opposite-sex DZ twins.

Regression analysis

The probands are chosen with scores that fall below a "cutoff" for what is considered "extreme", and regression is then used to predict the co-twin scores based on those of the probands and a term reflecting whether the twin pair is MZ (1.0) or DZ (0.5). The formula used for DF regression is: : where ''C'' = expected co-twin score, ''P'' = proband score, ''R'' = coefficient of relationship (0.5 for DZ twins, 1.0 for MZ twins), and ''K'' = regression constant. ''B''₁ represents a measure of co-twin relatedness separate from that of zygosity, while ''B''₂ can be converted into an estimate of the

of extreme scores on the trait. By comparing MZ and DZ co-twins in this manner, an estimate of "group heritability" is generated.

References

Regression analysis Behavioural genetics Quantitative genetics {{Statistics-stub