2 11 Shared Environment The shared environment is what is usually thought of by the environment Examples o Parents o Neighborhoods o SES socioeconomic status o School environment The shared environment makes sibling 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 Examples o Different peer groups o Different schools o Different experiences o Different prenatal environments Even for twins o E g a divorce Heritability Can also apply when siblings interpret or react to the same event differently Heritability h 2 refers to genetic influences on behaviors and traits Two categories if genetic influences o Broad heritability o Narrow heritability Captures effects of all sources of genetic influence Captures effects of only one source of genetic influence additive genetic influences 3 types of genetic influences o Additive genetic influence narrow The sum of each gene s contribution to phenotypic variance Capture the effect of the interactions between alleles of a single o Dominance broad o Epistasis gene broad Captures the effects of interactions between alleles from multiple Behavior Genetics genes BG method look at a specific trait or behavior o This is typically referred to as a phenotype Any measurable trait Decomposes the variance in a given phenotype into h 2 c 2 and e 2 o H 2 c 2 e 2 100 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 Shared environments 10 Nonshared environment 40 Heritability 50 2 13 Behavior Genetics There is nothing anti sociological about BG methods In fact BG methods are the most reliable way to estimate environmental effects o Allow us to directly and accurately estimate environmental influences on a given phenotype o Differentiates between shared and nonshared environmental influences How do we estimate these three effects Most researchers including criminologists look at only one child per family o Cant separate genetic from environmental influences Need to look at more that one child per family o Focus on children because that is when delinquency tens to peak Twin based design is the most common technique o We can compare monozygotic twins MZ to dizygotic twins DZ o Natural experiment Why MZ twins identical twins DZ twins fraternal twins Twin Studies Based on a fairly simple assumption o If MS 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 If DZ twins are just similar to one another as MZ twins then the environment has a greater influence The only reason the MZ twins would be more similar to one another would be because they share twice as much genetic material Critics of twin based research have argued that there are serious limitations of the Criticism of Twin Studies twin based design o Argue that these limitations result in artificially inflated h 2 estimates o Also result in artificially deflated c 2 and e 2 estimates Violation of the equal environments assumption EEA o MZ twins share environment that are more similar than DZ twins since they are genetically identical Presence of assortative mating o Mates seek out other mates with genomes that are similar to their own o Results in DZ twins that may share more than 50 of their dissenting DNA 2 18 Responses to Criticisms Violation 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 oh h 2 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 h 2 not increase them Likely artificially DEFLATED Other BG Techniques Limitations are not a concern o Additional techniques have been developed anyway Researchers have developed additional methods to separate genetic and environment influences o Adoption studies o MZ twins separated at birth MZA o Family studies Adoption Studies We can also estimate h 2 c 2 and e 2 with adoption studies We can compare the adoptees with their biological parents and 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 influences 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 of MZA twins Any resemblance between them would be due to genetics o Cannot be due to a shared environment o Remember that nonshared environments make siblings different from one 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 sibling cousin etc Also possible with biological parents o Use parent s score on the phenotype of interest as a gross risk measure Possible anytime we compare at least 2 people from the same household some sort of genetic component o Need to know level of genetic relatedness Finding from BG traits What do BG studies reveal about the heritability of behaviors and personality 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 three 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 3 Third Law A substantial portion of the variation in complex human behavior traits is not accounted for by the effects of