twin and adoption studies - research methods

• what happens in a twin study

  twins are compared to see if a behaviour is genetically based or due to environmental factors
• what is behaviour compared in

  groups on monozygotic twins and dizygotic twins
• how is similarity measured in twin studies

  with correlation or concordance
• what do we assume in twin studies

  both twin types grow up in the same environment so the only difference is how genetically related they are
• what do we expect to see if a behaviour is entirely genetic

  MZ twins to have a 100% concordance rate
• what do we expect to see if a behaviour is not entirely, but more genetic than environmental

  MZ twins to have higher concordance than DZ twins
• example of someone who carried out a twin study

  Gottesman and shields
• what did Gottesman and shields do

  investigated the genetic basis of schizophrenia
• Gottesman and shields procedure

  . found 57 pairs of twins where at least 1 had schizophrenia - from london, opportunity sample

  . contacted their co-twin

  . determined their zygosity with blood tests
• findings from Gottesman and shields

  . if one MZ twin had schizophrenia, their cotwin was 42x more likely to have it that the general population

  . if a DZ twin had it, their same sex twin was at least 9x more likely to have it too
• what happens in adoption studies

  adopted children are compared to their biological and adoptive relative
• what do you expect a behaviour to be if ppt more similar to biological relative

  more genetic
• what do you expect a behaviour to be if ppt more similar to adoptive relative

  more environmental
• example of someone who did an adoption study

  Heston
• what was Hestons aim

  to see how many adopted children of biological mothers with schizophrenia would also develop it
• Heston procedure


  . took 47 adults who were adopted at birth due to their mothers having schizophrenia, and 47 adults with mentally healthy parents

  . assessed them all to see who had developed schizophrenia
• what did previous data show about schizophrenia

  risk of developing schizophrenia if one parent had it was 10%
• findings from Heston


  10% of adopted adults had developed schizophrenia
• conclusion from Heston


  findings support influence of genes in schizophrenia
• evaluation - strengths of twin studies (2)

  1 - enables researchers to investigate the influence of genes due to assumption of shared environment

  2 - information for twin studies is usually taken from twin registries which has lots of data on thousands of twins = samples can be very large so data is more likely to be representative of population
• evaluation - weaknesses of twin studies (2)

  1 - may overestimate the genetic influence - MZ twins are more likely to share a more similar environment than DZ twins so some similarity in MZ results may be due to the environment rather then just genes

  2 - provide broad indication that a behaviour has genetic origin but can't identify the specific genes involved - more technology required to uncover the specific genetic influence
• evaluation - strengths of adoption studies (2)

  1 - have advantage of removing the extraneous variable of environment

  2 - useful in showing that twin studies overestimate genetic factors
• evaluation - weaknesses of adoption studies (2)

  1 - children may be adopted into families similar to their biological ones = environmental influences similar = apparent similarities with biological relatives may be due to coincidental environmental similarities rather than genes

  2 - people who adopt may not be representative of population = conclusions drawn about the effect of genes eg intelligence may not generalise to population as a whole
• overall evaluation

  weakness - impossible to seperate nature completely from nurture = possible confounding variables may affect the validity of findings

  weakness - sample sizes can be limited = generalisability difficult, may bot be able to recruit large and diverse groups of twins or adoptees