Key Study: The Minnesota Twin Study of Twins Reared Apart

Travis DixonBiological Psychology, Key Studies, Studies and Theories

When identical twins are separated at birth, it allows researchers to compare the effect of genetics with the environment. This gives helps us answer the question: "is behaviour nature or nurture?"

Understanding how and why twin studies are used is an important topic in biological psychology because they can give us important insights into the extent to which our behaviour is nature (genetics) or nurture. 


Is our behaviour a product of nature or nurture? In other words, are we born the way we are, or have we become this way through years of experiences? This is the classic question in psychology and one that Bouchard and his colleagues have attempted to answer since 1979 at their Minnesota Centre for Twin and Family Research (MICTFR).

In this time, over 100 sets of reared-apart twins and triplets have participated in the research. Rigorous analysis of scores of data has enabled the researchers to draw some strong conclusions. The following “study” is similar to the Vietnam Head Injury Study (read more here) in that it is one report from an ongoing longitudinal study. 

Twin and kinship studies are used to determine heritability – the extent to which variations in behaviour can be attributed to genetic factors. e.g. 50% heritability for IQ means that differences in IQ are half (50%) due to genetics, and the other half is environment.

Monozygotic (identical) twins have 100% of their DNA in common, whereas dizygotic (fraternal) twins have on average 50% of their DNA in common. Twin studies compare similarities in behaviour between MZ and DZ twins. Comparing similarities in behaviour between MZ and DZ twins allows researchers to see the extent to which these variations are based on genetics.

Group of kids playing with constructor

If two identical twins grow up to be completely different from one another, we can assume that their environments were more influential in their behaviour than genetics. However, if they grow up similar despite very different childhoods, we can . conclude genetics is more important. This is basically how twin studies work, although they are more scientific in their approach.

In adoption studies (like the one below), the can also compare MZT twins (monozygotic together twins) – those who have been raised in the same household and those who have been raised apart (MZA). This allows researchers to compare the variable of the environmental influences on behaviour (because genetics is a constant variable between these two groups).

The behaviour we will focus on in this particular review is intelligence (IQ).


The researchers for this study (and continual studies) don’t gather all their data at once, but continually, as they find participants to take part in the research. The participants come from mainly the UK and USA, but have also included Australia, Canada, China, New Zealand, Sweden and Germany.

They become involved either because:

  • the twins, a friend or family member finds out about the research
  • someone involved in the adoption process works with the MICTFR to put them in contact with the twin
  • Or…one twin becomes aware that they may have a twin somewhere and they contact the MICTFR and ask for their help in finding their separated twin.

Twin study by Bouchard et al. (1990)

For this study, the average age of the twins when they participate in this study was 41, which is important because most twin research prior to this focused on adolescents. The twins spent an average of 5 months together before being separated and reunited (on average) around 30 years of age.

Physical and psychological data was gathered in a number of different ways, which took around 50 hours. Methods were triangulated, either using researcher triangulation or methodological triangulation. For instance, when measuring IQ, three different IQ tests were used to gather and triangulate the data. And two different researchers conducted similar tests on the same participants. Controls were also established, like conducting the IQ tests on the twins at the same time but in different rooms under strict supervision by researchers.


Can you think of any ethical issues with this study?

To control for confounding variables in the environment, rigorous data was gathered on the childhood environments of the participants. For instance, a “Moos Family Environment Scale” was used to compare the impressions of the participants’ childhoods and a questionnaire was given to measure access to physical facilities, such as material possessions and cultural, mechanical and scientific goods. For instance, were their dictionaries, artworks and power tools in the house when they were growing up? This type of data enables researchers to draw conclusions regarding socio-economic environment of the families and where the participants grew up.


The analysis of the data revealed no significant difference between MZA twins (reared apart) and MZT twins (reared together) in regards to personality measures such as temperament, hobbies, interests, career pursuits or social attitudes.

Similar to previous research, this study also concluded that about 70% of differences between IQs in twins is due to genetic variation (70% heritability); the remaining 30% of difference is caused by environmental factors, which is similar to previous research.

There was also evidence from this study that suggested that twins that spend more time together after they are re-united are more similar. However, the data also suggested that it is the level of similarity between the twins that determines how much time they spend together, not the other way around. 

Through their analysis of genetic and environmental variance, Bouchard et. al. concluded that genetics are an important factor in determining behaviour, but environment is also important. In addition to other research that suggests IQ similarities between children and adults increase over time, the researchers conclude that it can be our genetics that determines our environmental experiences. For example, if we have a naturally introverted disposition due to genetics, this will influence our psychological and personal experiences in life. If we think about neuroplasticity (the brain’s ability to change as a result of experience), you should be able to see how our intelligence may be malleable and this malleability is caused by a combination of genetic and environmental factors.

Critical Thinking Questions

  • How do the results of this study show intelligence is influenced by genetics and/or our environment? (Application)
  • What are some relevant ethical considerations particular to this study? (Analysis)
  • What are the strengths and limitations of this longitudinal study? (Evaluation)


Bouchard, Thomas J, Jr. Lykken, David T. McGue, Matthew. Segal, Nancy L and Tellegen, Auke. “Sources of Human Psychological Differences: The Minnesota Study of Twins Researched Apart.” Sciences, New Series, Vol. 250 (1990), pp223-8. Accessed from


The film “Three Identical Strangers” provides us with an interesting first-hand look into the world of psychological studies on twins separated at birth (link).