Nature, nurture and behavior - Heritability
11 important questions on Nature, nurture and behavior - Heritability
Three laws of behavior genetics
First law--> All human behavior traits are heritable. All psychology is heritable.
Second law --> The effect of being raised in the same family is smaller than the effect of genes. C is smaller than A. The effect of the shared environment is usually smaller than the effect of the shared genes.
In some traits C is not there at all but A is always there
Third --> A substantial portion of the variation in compex human behavioral traits is not accounted for by the effect of genes or families.
Unique environmnet accounts for 50% of the differences.
For specific religion the heritability is near 0/ your religiousity /Specific language/ fluency
A heritability of 0 means that differences in genes do not account for the differences in which specific religion you believe in and that is 100% environmental determined. However there are genes involved in making a brain that wants to believe and have fate.
The same is true for your specific language.
Which language you speak is determined by the environment. But you do need genes to make vocal cords so you can speak.
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Adoptees study/family method
Adoptees study --> Start with a characteristic of the genetic parent and look if the adopted child has the same trait.
Adoptees family method --> Start with a characteristic of the adopted child and look if the biological or adoptive parents have the same trait.
Families of twins method
Investigate the children of the twins when they are adults.
Families of male identical twins, nephews are as related genetically to their twin uncle as they are to their own father.
Cousins are as closely related to each other as half siblings are.
Rearing environment within the family can be disentangled from genetic influences for examening intergenerational transmission
Extenstion of families of twins methods
Combination of twins and their children and a sample of children who are twins and their parents.
Allows the effect of parent on children and of children on parent to be examined.
Rough estimate of heritability in a twin study can be made by
Doubling the difference between the identical and fraternal twin correlations 2(rmz-rdz). The difference in their correlations reflects half of the genetic effect and is doubled to estimate heritability.
So as correlation is 0,86 for identical and for faternal twins 0,57. Doubling the difference between these gives a heritability about 60% 0,86x2 - 0,57x2= 1,72-1,14= 0,58% heritability correlation
We can use The liability threshold model to translate
Concordance into correlation on the assumption that a continuum of genetic risk underlies the dichotomous diagnosis.
For shizophrenia means that the liability correlation for identical twins is 0.86 and for fraternal twins 0,57. Doubling the difference between these gives a heritability about 60%.
Shared environmental influences
Nongenetic influences that make family members similar.
Factors will be shared environmental influences only if they result in greater similarity among individuals living in the same houshold and if they do not vary as a function of genetic relatedness
When fraternal twins are as similar identical twins
When environmnetal siblings are as alike as genetic siblings.
Nonshared environmental influences
Nongenetic influences that are uncorrelated for family members.
Identical twins living in the same household share all of their genes and share their environment, the only thing that can account for differences within pairs of identical twins is nonshared environmental influences.
Estimating shared and nonshared environmental influences
Shared environment is estimated as family resemblance not explained by genetics. A direct test of shared environment is resemblance among adoptive relatives.
Nonshared environment is the rest of the variance --> Variance not explained by genetics or by shared environment. A direct test of nonshared environment is the differences among identical twins reared together
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