Monozygotic (identical) and dizygotic (fraternal) twins who were separated early in life and raised in different environments provide the simplest and most powerful method for studying the impact of nurture (environment) and nature (genetics) on human characteristics. Monozygotic (identical) twins share the same genetic makeup. If they are raised in different environments, researchers are able to clearly examine the impact of environment, since genetics are identical.
Sample
"More than 100 sets of twins or triplets from across the United States and United Kingdom have participated in the Minnesota Study of Twins Reared Apart since it began in 1979."
"The sample consists of adult twins, separated very early in life, reared apart during their formative years, and reunited as adults."
Methods of Study
Participants completed approximately 50 hours of medical and psychological assessment. This included:
Four personality trait inventories
Three occupational interest inventories
Two mental ability assessments (one individual and one administered as a group)
Intelligence Quotient (IQ) test, administered by separate examiners
Life history interview
Psychiatric interview
Sexual life interview
Cultural and social questionnaires
Independent assessment of twins’ environments that may have impacted psychological development
Findings
70% of IQ differences between twins were traced to genetic factors
Twins were about equally similar on most physiological and psychological traits
No significant differences in cognitive abilities or personality were identified
Conclusions
"The study of these reared apart twins has led to two general and seemingly remarkable conclusions concerning the sources of the psychological differences-behavioral variation-between people: (i) genetic factors exert a pronounced and pervasive influence on behavioral variability, and (ii) the effect of being reared in the same home is negligible for many psychological traits."
"Psychologists have been surprised by the evidence that being reared by the same parents in the same physical environment does not, on average, make siblings more alike as adults than they would have been if reared separately in adoptive homes…On multiple measures of personality and temperament, occupational and leisure time interests, and social attitudes, monozygotic twins reared apart are about as similar as are monozygotic twins reared together."

In one paragraph, describe the research methods used in this study and why you believe they were chosen. Be sure to use key terms such as cross-sectional research, longitudinal research, data collection, observation, case studies, questionnaires, and experimentation, as they apply. Your answer should include at least two of these key terms. In one paragraph, discuss the research findings. Does the study support genetics (nature) or environment (nurture) as a stronger influence on human development? Use information from the article and the lesson to support your response.

Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics and in content fields, from biology to psychology. Twin studies are part of the broader methodology used in behavior genetics, which uses all data that are genetically informative – siblings studies, adoption studies, pedigree, etc. These studies have been used to track traits ranging from personal behavior to the presentation of severe mental illnesses such as schizophrenia.

Twins are a valuable source for observation because they allow the study of environmental influence and varying genetic makeup: "identical" or monozygotic (MZ) twins share essentially 100% of their genes, which means that most differences between the twins (such as height, susceptibility to boredom, intelligence, depression, etc.) are due to experiences that one twin has but not the other twin.[1] "Fraternal" or dizygotic (DZ) twins share only about 50% of their genes, the same as any other sibling. Twins also share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) because they are born into the same family. The presence of a given genetic or phenotypic trait in only one member of a pair of twins (called discordance) provides a powerful window into environmental effects on such a trait.

Twins are also useful in showing the importance of the unique environment (specific to one twin or the other) when studying trait presentation. Changes in the unique environment can stem from an event or occurrence that has only affected one twin. This could range from a head injury or a birth defect that one twin has sustained while the other remains healthy.

The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (which is found for most traits), this implicates that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior.