Genetic explanation of aggression (Twin and adoption studies (Twin studies…
Genetic explanation of aggression
Twin and adoption studies
MZ and DZ twins are born at the same time and share the same environment, but MZ twins have identical DNA, whereas DZ twins only share 50% which helps identify how genes could influence characteristics.
There should not be significant environmental differences with regards to the treatment of twins. They aren't treated separately.
MZ twins are the same sex and identical so they will be treated alike more than DZ twins.
Epigenetic modification (over time different environmental influences affect which genes are switched on and off) can have an effect on the findings.
Separate genes from the environment so the two can be tested.
Studies are longitudinal (conducted over a long period of time) so developmental trends can be identified.
Environment of the adoptive families is not different as it could be from the biological family's.
Children requiring adoption are often placed with families similar to their own, so differences are minimal.
Look for correlations in aggressive behaviour between children raised by non-biological parents, and aggression in the biological parents
Hutchings & Medick (1973)
14,000 adoptions in Denmark
Measured number of convictions for criminal violence
Significant positive correlation found between adoptees and biological parents for criminal conviction
No significant correlation between adoptee and adoptive parents
Come from one fertilised egg
DNA is 100% shared, therefore always the same sex
Inherited by both twins
No characteristic is entirely genetic as environment is a depending factor
May become less identical over time due to environmental factors
Come from two different fertilised eggs
DNA is not 100% same, similar of any sibling pair
Share an inherited characteristic to an extent, not as much as MZ twins
Compare certain characteristics possessed by MZ and DZ twins to see if genes or environment influence whether they share characteristics.
The extent to which the behaviour is the same between twins is the concordance rate.
A gene that may be responsible for aggressive behaviour
Instructs the brain to regulate aggression
MAO-L, the variant, doesn't work as effectively
Found on the X chromosome so men are more likely to be affected by it
If women have MAO-L on one chromosome, they might still have the normal MAO-A on their other X chromosome
Carl Frydman argues this gene makes people willing to take risks but also improves ability to look at critical situations.
83 young men competed at gambling $25 in 140 games
Each had to choose between a sure option and a risky option
People with MAO-L were more likely to take financial risks that benefitted them rather than pointless risk taking
Evolution favours those who take risks, but only if they gain something
The most aggressive strains of a certain species can be interbred and the offspring can be measured for aggressive behaviour.
Mice classified as either 'aggressive' or 'docile'
The 2 strains were interbred and 'fostered'
By 19th generation, 52% of 'aggressive' mice's offspring showed biting behaviour compared 5% of non-aggressive
MAOA is one gene that has been identified that can contribute to increased risk for aggression in some people under some circumstances
MAOA does not act alone but together with many other genes and hormones to produce particular outcomes as complex as aggression