Evidence of a Genetic Link to Criminal Behavior: Level #1 The earliest evidence came from family studies. These studies found that if a father was criminal then there was an increased chance the child would be criminal (proband). Numerous studies, conducted across continents, now document this fact. Having criminal parents substantially increases the likelihood their child will be criminal.
Family Studies Continued: Farrington found that 10% of all families in his London sample produced 90% of all serious delinquents. Strong intergenerational continuity. These families have multiple problems associated with crime and drug use, including divorce and low SES. These families also tend to produce above average numbers of offspring.
The Next Level of Evidence: Twin Studies Mz twins=genetic duplicates Dz twins=fraternal twins (can be male and female) Evidence comes from all over the world and from independent researchers Establishes “concordance rates” Concordance=similarity in traits across levels of genetic association Thus, if one twin is anxious and then the other is also found to be anxious, they are concordant on anxiety
Notes on Twin Studies: Early studies suffered methodological problems….namely that twins were reared in the same environment. Even so, the evidence still followed genetic expectations. Modern studies have examined twins reared apart. These are some of the most conservative studies available.
Summary of the Evidence: IQ RelatednessActualGeneticallyEnvironmentallyPredicted MZ twins reared together.861.01.0 MZ Twins reared apart.721.00.0 DZ twins reared together.60.51.0 Biological Sibs together.47.51.0 ½ sibs together.36.251.0 Adoptive Sibs together.34.001.0 Sibs reared apart.24.50.0 Bouchard and McGue, Science, 1981, p. 1057 ***Notice the fairly strong concordance rates based on level of genetic relatedness!
More Evidence From Robert Plomin: AbilityMzDz Verbal Comprehension.78.59 Verbal Fluency.67.52 Reasoning.74.50 Spatial Visualization.64.41 Perceptual Speed.70.47 English Usage.72.52 Math.71.51 Social Studies.69.52 Science.64.45 Notice the wide range of abilities that are genetically related.
What About Juvenile Delinquency? For Juvenile Delinquency: Gottesman, Carey & Hanson, 1983 87% concordant MZ 72% concordant DZ Rowe, 1983 .71 Mz to.47 Dz
And Adult Criminality? Eight studies yield an average concordance rate of: .69 for Mz twins .33 for Dz twins
Adoption Studies: The Third Layer of Evidence Adoption data allow us to examine the effects of environment on human development. Typical adoption studies measure the traits/behaviors of the offspring and of the adopted and biological parents. These studies indicate that adopted children behave more like their biological parents than their adoptive parents. Evidence also shows important interaction effects between having criminal biological parents and being adopted into a criminal household. Again, demonstrates the effects of environment on genetic expression.
Conviction Patterns for Adoptees by Level of Genetic Association: ** Compare Biological Father and Against Adoptive Parents……………
Summary Thus Far: Clear and consistent evidence showing a genetic link to various human traits and behaviors. All sources of data (family studies, twin studies, and adoption studies) converge on this conclusion. The evidence comes from various countries and from independent researchers.
Another Layer: Behavioral Genetics Purpose of BG is to account for the percentage of variance attributable to genes and/or environment. Uses “genetically informed” methodology that includes various levels of genetic relatedness. Relies heavily on quantitative genetic models that partition variance in a trait.
BG Continued: Draw on concept of heritability: H=“a quantification (from 0 to 1) indicating the extent to which variance in a trait is due to genetic influences (Walsh, 2002). Variance in traits, like aggressiveness, occurs through the effects of polymorphism (differences in allelic combinations at chromosomal loci that make us each different). Thus, we “inherit” DNA for a toe, a brain, and fingers. However, heritability refers to variance in traits not shared by everybody.
And Some More: Thus, BG dissect behavior into Heritability (genes) Shared Environment (those things that make us similar—like family) Non-shared Environment (those things that make us different—like peers) Genes Shared Nonshared
Evidence From BG Studies: Genetic influences are found on almost every possible outcome, even religiosity! For most traits, nonshared environmental influences account for much of the variance left over after genetic variance is removed. The effect of genes grows STRONGER over the life-course! This is true for crime and for IQ Share environmental influences (think parents) do NOT tend to account for much of the variance in most outcomes. Kids brought up in the SAME household turn out very different. Why? Nonshared environment (peer) may play a larger role than parents Parents pass on their genes to their kids. People in the same environment experience it differently! Even parents tend to select their favorite kids for better treatment.
Conclusions: Taken together, there is consistent and undeniable evidence liking genes to a variety of crime-related traits. On whole, criminality is ~60% heritable. Genetic effects appear to dwarf other environmental influences—namely parents. Genetic expression INCREASES over time— that is, individual traits operate on their environment.