brendgen (twin) contemporary study

• Twin studies


  A study where a trait/ behaviour is compared between twins to see how similar they are
• Concordance Rate


  The degree to which two people are similar on a particular trait/behaviour
• Monozygotic (MZ) twins


  • Identical Twins
  • Single egg fertilized by single sperm, then splits in two
• Dizygotic (DZ) twins


  • Fraternal Twins
  • Separate eggs fertilized by separate sperm
• If a trait/ behaviour is entirely genetic, then MZ twins would show 100% concordance
• If a trait/ behaviour is influenced more by genes than environment, then we would expect MZ twins to show a higher concordance rate than DZ twins
• Strengths of twin studies

  • Allow researchers to measure the influence of genes because each set of twins share an environment, so the only difference between MZ & DZ twins is genetics (MZ 100%, DZ 50%)
  • Samples usually large because data is taken from twin databases which contain info on many variables (mental health, addiction etc.)
  • Samples can be representative due to being large
• Weaknesses of twin studies

  • Twin studies assume twins grow up in the same environment, but this is wrong because there are small differences between even identical twins
  • These differences mean there are actually TWO environmental influences: shared and nonshared
  • Twin studies can't identify the specific genes involved just a broad indication of whether a behaviour is genetic or environmental
• Adoption studies


  The main ways to study nature versus nurture has been to use twin or adoption studies
• Strengths of adoption studies

  • Enables researchers to investigate the influence of genes because it is assumed that both MZ and DZ co-twins share the same environments. The only difference between the two groups of participants is genetic
  • Although twins are unusual, information for twin studies is often taken from twin registries (such as the MTFS used in Ludeke et al's study on facing page). These hold data on thousands of twins and also contain information about many variables. This means that the sample is large and the data is likely to be representative
• Weaknesses of adoption studies

  • Children may be adopted to families similar to their biological families (called "selective placement") and therefore environmental influences may be similar
  • People who adopt other people's biological offspring are unusual, so are unlikely to be representative of the population. They tend to be better educated, have lower rates of mental illness and come from higher socioeconomic groups
• Concordance


  The degree to which two people are similar on a particular trait/behaviour
• Heritability


  The degree to which a trait is inherited
• Twin study of social aggression

  1. Researchers used a twin study design to calculate correlations between MZ and DZ twins who were rated for socially and physically aggressive behaviour

  2. Teacher ratings: Kindergarten teachers rated the social and physical aggression of each child

  3. Peer ratings: The levels of the twins' social and physical aggression were also rated by their classmates
• Only 20% (teacher ratings) to 23% (peer ratings) of social aggression was explained by genetic factors
• Physical aggression was mostly explained by heritable (genetic) factors and partly by non-shared environmental influences. In contrast social aggression was mostly explained by non-shared environmental factors
• A moderate but significant correlation between social and physical aggression, explained mostly by overlapping genetic influences
• Statistical testing showed that high physical aggression led to high social aggression, but the opposite was not true (for teacher and peer ratings)