BG: Current Definitions

Current Definitions in BG
James Brody
Revised 12/25/02

Shaun Purcell has an introduction to statistical mehtods in behavioral genetics in the Appendix to Plomin, et al., 2001.

1) Order of similarity in outcomes follows the order of similarity in genes. (1) That is, beavers resemble beavers more than they resemble humans. Monozygotic twins are more similar than dizygotic, adoptees are no more similar to their adoptive family than to a randomly chosen stranger.

2) Outcomes = f (Genes) x (Environments). The relationship is multiplicative, neither element alone produces much of anything.

3) Phenotypic variation = genetic variation + environmental variation = 1.00. As one component increases, the contribution from the other decreases.
4) Genetic variation = additive (genes sum in their effects) or interactive (epistatic).

5) Environmental variation = shared (SE) plus nonshared (NSE, also called "unique environment"). SE makes us act alike and typically contributes about 2% or less of long term results, NSE makes us (allows us to be?) different and contributes between 30-40% (Plomin, 1994; Pinker, 2002).

6) Gene-Environment correlation, GE Interaction, and error contribute the rest after we subtracts contributions from heritability, environmental variance, and error. GE correlation: difficult idea to apply in human dilemmas, simple to apply in cross-species comparisons. Bird genes are usually found in trees. GE Interaction: genetic propensities for impulsive or antisocial behavior are most evident in settings where other individuals express impulsive and antisocial behavior. Error, assessed when the same assessment is made twice of the same individual, is often about 0.20.

7) Heritability is a measure of variation attributable to genetic variation. No variation in outcomes = no heritability. (Example: the number of eyes that we have is inherited but with zero heritability!)

8) Strong heritability does not imply that environment has no influence. Example: PKU is a genetic condition in the metabolism of phenylalanine. Diets low in phenylalanine prevent the genetic effects.

Notes & References
Pinker, S. (2002) The Blank Slate: The Modern Denial of Human Nature. NY: Viking.
Plomin, R. (1994) Genetics and Experience: The Interplay between Nature and Nurture. Thousand Oaks, CA: Sage.
Plomin, R., DeFries J, McClearn G, & McGuffin, P. (2000) Behavioral Genetics (4th ed.) NY: Worth.
1) Dunn, J. and Plomin, R. (1990) Separate Lives: Why Siblings Are So Different. NY: Basic., Cohen, D. (1994) Out of the Blue: Depression and Human Nature. NY: Norton. Cohen, D. (1999) Stranger in the Nest: Do Parents Really Shape Their Child's Personality, Intelligence, or Character? NY: Wiley. Harris, J. (1998) The Nurture Assumption: Why Children Turn Out the Way They Do. NY: Free Press.

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Copyright, James Brody, 2002, all rights reserved.

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