Twin and adoption studies

• what is the purpose of studying twins and adopted children?

  To examine the causes of characteristics that might be genetically controlled
• Twin studies - MZ twins

  • identdical twins are monozygotic twins - they come from one fertailised egg where their DNA is 100% the same
  • is a characteristic is genetic then MZ twins should both show that characteristic
  • however there is always some influence from the environment
• what is epigenetic modification?

  This is how over time different environmental influences affcet which genes are switched on and off

  usually younger identical (MZ) twins have few epigenetic differences but as they get older these may increase

  • linking to nature v nurture debate in that it is unlikely that any characteristic comes wholly from either nature or nurture
• Twin studies- DZ twins

  • they do not come from the same egg
  • the DNA of DZ twins is only as similar as any siblings- not 100%
  • if a characteristic is genetic DZ twins would be expected to share the characteristic to an extent
• what do twin studies do?

  They compare MZ and DZ twins on certain characteristics to see if there are differences between the MZ twins and the characteristic and between DZ twins and the characteristic.

  When both twins share a characteristic there is said to be a concordance rate
• Strengths of twin studies:

  • Both MZ and DZ twin babies are born at the same time and share a similar environment but MZ twins share the EXACT same DNA compared to DZ twins that share 50% of genes- this allows us to see the influence of genes on behaviour
• Weaknesses with twin studies:

  • MZ twins being of the same sex and likely to look identical will be treated more alike than DZ twins which means that their environment are not the same
  • epigenetic modification need to be taken into account- MZ twins do experience differences which may mean over time they become different - it is DIFFICULT to say that a characteristic is caused by genes just because MZ twins share it and DZ twins do not share it
• TWIN STUDY- Lacourse 2014- sample

  The study was carried out in Canada using 254 MZ and 413 DZ twins that were part of a longitudinal study
• TWIN STUDY- Lacourse 2014 aim

  To look at genetic and environmental issues underlaying the development of physical aggression from infancy to when a child is at school age
• TWIN STUDY- Lacourse 2014 procedure

  Used both types of twins, and information about their physical aggression was collected by the Quebec Newborn Twin study at around 19 months, 31 months and 50 months.

  Physical aggression was measured as a part of a questionnaire using a likert scale given to mothers, with three times measuring aggression.

  Such as how many times a child hits,bites, kicks being one
• TWIN STUDY- Lacourse 2014 findings

  Differences between MZ and DZ twins showed that MZ twins were more likely to both show the same scores than the DZ twins.

  MZ- correlated at 0.68 to 0.72

  DZ- correlated at 0.32 to 0.47

  mz twins are more likely to be similar in terms of aggression and in the way they rise between the three age groups
• TWIN STUDY- Lacourse 2014 conclusion

  The early onset of physical aggression was explained by genetic factors and these declined over time
• Evaluating Lacourse 2014

  • The size of the sample gathered lots of information- more representative and has population validity
  • used self report from mothers- may not be honest creating social desirability bias
  • correlational study - so we cannot say that genes cause aggression there is just a link as additional factors may have contributed
• what is the role of adoption studies ?

  these are carried out because the environment of adopted children is not the same as that of their biological families
• Strengths of adoption studies:

  • good way of separating the genes from the environment- similarities cannot be because of environmental factors
  • these studies can be longitudinal which allows development trends to be studied, where the same child can be followed as they develop
• Weaknesses of adoption studies:

  • likely that only certain types of family are accepted as adopters of children so adopting families are likely to be similar to one another
  • children tend to be placed in families like their own so their birth family and adopted may share similarities which limits how far environment is different
• studying MZ twins reared apart:

  These look at MZ twins that have been separated at birth and brought up apart apart- controlling that they have 100% of same DNA and if they share similar characteristics it is likely that this due to genes

  +environmental conditions are controlled

  +no other way of finding identical human DNA and having two different environments

  -there are not large numbers from which to draw conclusions and difficult to find
• ADOPTION STUDY- Leve 2010 aim

  To look at family relationships and features in such relationships that affect genetic influences from infancy
• ADOPTION STUDY- Leve 2010 procedure

  Used an adoptive triad- which is the birth parents the adoptive parents and the child. Enables the researcher to see which genetic influences and which environmental influences affected one another and how.

  360 linked sets of adoptive triads- Leve wanted to assess the participants in the school entry periods, measuring executive function, early literacy skills and cortisol reactivity

  Assessment was a questionnaire and interviews for both sets of parents and the children- saliva cortisol collection from all participants
• ADOPTION STUDY- Leve 2010 findings

  Their findings indicated that as early as two and a half, children can show precursors to aggressive behaviour, such as not shifting their attention away from a frustrating event. They also found that the adoptive mothers more than fathers had influence in the ability of the child to regulate their emotions, suggesting that there are environmental factors involved in children’s aggressive behaviour.
• evaluating Leve 2010

  • size of the sample could mean its representative
  • researchers used various methods of gathering data and several times- collected cortisol 6 times shows reliability
  • hard to show cause and effect- natural experiment