Behavior Genetics BG distinguishes between two different types of environments Shared environment o Makes siblings similar Nonshared environment o Makes siblings different Shared Environment The shared environment is what is usually thought of by the environment o Examples Parents Neighborhoods SES socio economic status School environment Makes siblings more similar to each other Why o Both siblings experience the same environment o Has the same effect on all children from the same household Nonshared Environment The nonshared environment is very different than the shared environment The nonshared environment makes siblings different from each other Different environments will result in different influences on the trait of interest o Results in differences between siblings from the same household o Examples Different peer groups Different schools Different experiences Different prenatal environments Even for twins Can also apply when siblings interpret or react to the same event differently o E g a divorce Heritability Heritability H2 refers to genetic influences on behaviors and traits Two categories of genetic influences o Broad Heritability o Narrow Hertitability Captures effects all sources of genetic influence Captures effects of only one source of genetic influence additive genetic influences 3 types of genetic influence 1 Additive genetic influence narrow The sum of each gene s contribution to phenotypic variance 2 Dominance broad Captures the effects of interactions between alleles of a single gene 3 Epistasis broad Behavior Genetics genes Captures the effects of interactions between alleles from multiple BG method look at specific trait or behavior o This is typically referred to as a phenotype Any measureable trait Decompose the variance in a given phenotype into heritability h2 shared environment c2 and nonshared environment e2 o H2 c2 e2 100 of the variance in the examined phenotype Take for example the phenotype of antisocial behavior o Using behavioral genetics methods we can divide this trait up into three pieces 1 Shared environment 10 2 Nonshared environment 40 3 Heritability 50 Behavior Genetics There is nothing anti sociological about BG methods In fact BG methods are the most reliable way to estimate environmental effects Allow us to directly and accurately estimate environmental influences on a given phenotype Differentiates between shared and nonshared environmental influences How do we estimate these three effects Magic Most researchers including criminologists look at only one child per family Can t separate genetic from environmental influences Twin based design is the most common technique We can compare monozygotic twins MZ to dizygotic twins DZ Natural experiment Why o MZ twins identical twins o DZ twins fraternal twins Twin Studies Based on a fairly simple assumption o If MZ twins are more similar to each other than DZ twins then genes have a greater influence What else could account for this finding Why not the shared environment o If DZ twins are just as similar to one another as MZ twins then the environment has a greater influence The only reason that MZ twins would be more similar to one another would be because they share twice as much genetic material Criticisms of Twin Studies twin based design Critics of twin based research have argued that there are serious limitations of the o Argue that these limitations result in artificially inflated h2 estimates o Also result in artificially deflated c2 and e2 estimates 1 Violation of the equal environments assumption EEA o MZ twins share environments that are more similar than DZ twins since they are genetically identical 2 Presence of assortative mating o Mates seek out other mates with genomes that are similar to their own o Results in DZ twins that share more than 50 of their dissenting DNA Responses to Criticisms Violations of the EEA o Studies have examined misclassified MZ DZ twin pairs E g classified as a DZ pair but is actually an MZ pair o No support for inflated estimates of h2 Assortative mating o Studies have found support for assortative mating o Increases the genetic similarity between non MZ siblings o Works to DECREASE estimates of h2 not increase them Likely artificially DELFATED Other BG techniques Limitations are not a concern o Additional techniques have been developed anyway Researches have developed additional methods to separate genetic and environment influences 1 Adoption Studies 2 MZ twins separated at birth MZA 3 Family studies Adoption Studies We can also estimate h2 c2 and e2 with adoption studies We can compare the adoptees with their biological parents with their adopted parents If the adoptee more closely resembles their biological parents then genes influence the examined phenotype If the adoptee more closely resembles their adopted parents then the environment has a stronger influence on the examined phenotype MZ Twins Separated at Birth We can also examine MZ twins who were separated at birth MZA o These are rare samples but some studies have identified an impressive number Any resemblance between them would be due to genetics o No shared environments o Remember that nonshared environments make siblings different from one of MZA twins another Family Studies Logic of the twin based design can be extended to other family members as well o Most common use is with non twin sibling pairs Full siblings half siblings cousin etc Also possible with biological parents o Use parent s score on the phenotype of interest as a gross genetic risk measure Possible anytime we compare at least 2 people from the same household o Need to know level of genetic relatedness Finds from BG What do BG studies reveal about the heritability of behaviors and personality traits o Findings are relatively consistent across different methodologies o Indicates that the results from any one method are valid and reliable Most behaviors and personality traits are highly heritable 50 90 o 50 represents a proportion and can be converted to a percentage e g 50 The shared environment has very little effect The nonshared environment is important Findings are so robust they have been developed into formally written laws 3 Laws of Behavior Genetics 1 First Law All human behavioral traits are heritable 2 Second Law The effect of being raised in the same family is smaller than the effect of genes Behavior Genetics 3 Third Law A substantial portion of the vibration in complex human behavior traits is not