What was Bouchard’s rationale for using identical twins reared apart from birth in his study of the relative contributions of nature and nurture to intelligence and personality? What conclusions were supported by his research?
Galton was the first one to use twins in his research. Then, Thomas Bouchard conducted his research with the twins, as Galton’s research showed wide acceptability. Bouchard researched genetic influence on personality and physical characteristics, and intelligence. He also used other four primary comparison mentioned below:
1. Twins reared together, Dizygotic, or fraternal (DZT)
2. Twins reared apart, Dizygotic, or fraternal (DZA)
3. Twins reared together, Monozygotic, or identical (MZT)
4. Twins reared apart, Monozygotic, or identical (MZA)
Bouchard explained the fact that DZTs and MZTs would show high correlation, if studied as per experience determining personality and intelligence, whereas DZAs and MZAs would not. Further, if the genetics determine personality and intelligence, MZTs and MZAs would result in high correlation, whereas DZTs and DZAs will show modest. This is because the twins (monozygotic) are separated from each other by birth.
Bouchard also explained the fact that monozygotic shares a wide variety of similarity including finger prints and heights. Further, he states that as per intelligence heritability (indication of the deviation on trait, which is capable of being attributed to genetics) is a complex phenomenon obtained from coefficients of correlation. MZA twins shows 70% of intelligence heritability.
As per personality characteristics, the monozygotic twins present highest level of similarity. This further helped in proving that family environment contributes less to the personality, as seen in the twins reared apart. So, the conclusion made was that genetics provides major contribution towards intelligence and personality, but is not a sole contributor.