ac 2.1 + 3.2 - genetic theories (bio) Flashcards
what are the genetic theories?
twin and adoption studies and Jacob’s XYY study
what are twin studies used for?
they are used to look at the role of genes in determining certain behaviours or traits, researchers use twin studies when they are interested in two types of twins
what are MZ twins?
- monozygotic twins (identical)
- share 100% of their DNA
- from one fertilised egg that split in half
what are DZ twins?
- dizygotic twins (non-identical)
- share 50% of their DNA
- from two fertilised eggs
what do twin studies use concordance rates for?
to compare MZ concordance rates for a particular behaviour or trait - such as the tendency to commit a crime - with DZ concordance rated for the same behaviour or trait in order to see whether behaviour (like criminality) is more due to nature or nurture
what do different concordance rates mean?
- 100% for MZ twins - criminality is completely due to nature. if one twin is a criminal, the other will be too
- less than 100% - criminal behaviour must be partly due to nurture and not just nature
- higher for DZ twins - criminal behaviour is more due to nurture than nature
- MZ=DZ concordance rate - criminal behaviour us more due to environment
Johannes Lange (1929) study
looked at 13 MZ twins and 17 DZ twins. found that MZ twins showed a much higher degree of concordance than DZ twins for criminal behaviour. 10/13 MZ twins had both served time in prison, whereas only 2/17 DZ twins had this concordance rate
Christiansen’s study in Denmark (1977)
- studied 3,568 sets of twins
- the main results were: male MZ twins had a 35% concordance rate, whereas male DZ twins had a 13% concordance rate. female MZ twins had a 21% concordance rate, whereas female DZ twins had a 8% concordance rate
- results indicate a degree of inheritance, however the concordance rates re still quite low for MZ twins, suggesting that environment must play a large part in criminal behaviour
strengths of twin studies
- naturalistic experiment. variable is naturally occurring and there is no manipulation at the hands of the researcher. therefore, results will be more accurate to how genetics may affect criminality in real life for twins.
- Christiansen supports. studies 3,568 sets of twins and found a 35% concordance rate for male MZ twins, whereas this was 13% for male DZ twins. therefore, adds credibility as results show there must be some kind of genetic influence to criminality.
weaknesses of twin studies
- results are easily criticised, if criminal behaviour was entirely genetic then the concordance rate for MZ twins would be 100% and this has never happened. for example, in Christiansen’s study concordance rates are only 35% for male MZ twins which is quite low. therefore, this shows that there is no criminal gene and that there must be some sort of environmental influence on criminality.
- low generalisability. twins only take up about 1.5% of the environment, with 1/3 of these being MZ twins. therefore, results for how genetics can influence criminality in terms of twin studies cannot be generalised to the wider population.
- low internal validity due to the possibility of confounding variables. twins re usually brought up in the same environment, treated the same, and could even have the same social groups. therefore, this limits the credibility as criminality could be due to their shared upbringing and environment rather than their genes.
what are adoption studies used for?
to separate the effects of genes from environmental factors as twin studies don’t allow for this. adoption studies involve looking at an individual and their genetically related relatives in comparison to adoptive relatives who are genetically different but share the same environment as the individual
concordance ages in adoption studies
- if genetically related relatives show a higher degree of concordance for a condition or behaviour than the adoptive relatives do then this suggests that genetic factors are important in causation (nature)
- if concordance rates are higher in adoptive relatives then this suggests environmental factors are more important (nurture)
Mednick’s study (1984)
- looked at 14,000 adopted males born between 1924 - 1947 in Denmark
- he compared criminal records of the adopted males with the criminal records of both their biological and adoptive parents
- results: 20% concordance when only biological parents had convictions, 14.7% concordance when only adoptive parents had convictions and 20% when both had convictions
- results suggest that there is a strong genetic component, however they also suggest that there is an environmental influence as the rates are low
strengths of adoption studies
- overcome problem with twin studies by allowing us to separate the roles of nature and nurture. adoption studies use blood relatives who were brought up in different environments, e.g. bio parent and their adopted child. therefore, results on how genes may influence criminal behaviour will be more accurate.
- there is support from Mednick. looked at 14,000 adopted males and compared there criminal records with that of there bio and adopted parents, found higher concordance rate for bio parents (20%) than for adoptive parents (14.7%). therefore, this supports the idea that criminality may be a result of genes.
weaknesses of adoption studies
- lack validity as a cause and effect between genes and criminality can’t truly be established. correlations show a relationship between co-variables and not causation. therefore, there is decreased accuracy in the results of how genes may influence criminal behaviour as there may be a third unknown variable.
- if children aren’t adopted from a very young age then this may affect results. they may be more influenced by their bio parents and their early environment with them if they are not adopted from birth. therefore, this decreases the credibility of how genes may influence criminality in terms of adoption studies.
- Mednick’s research criticises the idea of criminality being completely due to nature. the highest concordance rate was when both bio and adoptive parents had a criminal record (24%), whereas this was 20% for bio only and 14.7% for adoptive only. therefore, this suggests that both nature and nurture can influence criminal behaviour rather than it being completely due to genes.
physical characteristics of XYY syndrome
- bad acne (in adolescence)
- pinky fingers curve inwards
- weaker muscle tone
- wildly spaced arms
- taller than average height
psychological characteristics of XYY syndrome
- behavioural issues
- anxiety
- risk of ADHD
- hyperactivity
- lower IQ (10-15 points lower than average)
- depression
what is XYY syndrome?
- happens when a male passes on an extra Y chromosome
- only occurs in males
- there is no cure but there are intervention services for specific behaviours
how common is XYY syndrome in the general population?
rare, affects about 1 in 1,000 male births
how common is XYY syndrome in the prison population?
found in about 15 in 1,000 males. more common than in the general population but still quite rare
Jacob et al (1956)
found a higher proportion of XYY in the prison population (15/1000) then in the general population (1/1000). however, Jacob’s research only studies inmates of a secure psychiatric hospital
strengths of XYY theory
- there is research to support XYY theory as an explanation for criminality. Jacob et al found that a significant number of men in prison had XYY chromosomes instead of the normal XY. therefore, this suggests a possible relationship between the XYY pattern and criminality.
- there is research to support XYY theory as an explanation for criminality. Adler et al indicated that it is possible that aggressive and violent behaviour is at least partly determined by genetic factors and a psychological characteristics of XYY is behavioural problems. therefore, this supports the idea that criminality may be a cause of genetics.
- there is research to support the XYY theory as an explanation of criminality. Price and Whatmore found some link between XYY and property crime. this suggests that there may be a link between this chromosome pattern and specific forms of criminal behaviour, supporting the idea that genes can influence criminality.
weaknesses of XYY theory
- there is low internal validity. evidence between XYY and criminal behaviour is only correlational meaning that a cause and effect cannot be established. therefore, this decreases the credibility of results as we can’t truly say that XYY leads to criminality due to the fact there may be another unknown variable.
- this theory is androcentric. XYY can only be used to explain male criminality as females cannot have XYY syndrome. therefore, this theory cannot be generalised to the wider population/all genders, meaning it can’t provide a comprehensive explanation.
- XYY theory is reductionist. it primarily focuses on genetic influence and overlooks social and environmental factors. therefore, this theory is oversimplified and lacks credibility in terms of being an explanation of criminal behaviour.
