Repost: History of Psychology: Identical Twins- TWO Individuals, ONE Personality?
In Dr. Andi Lassiter’s History and Systems of Psychology course, students complete a project on the local history of psychology. The goal of the assignment is to research some aspect of psychology in the upper Midwest, such as local asylums or people / events related to psychology. Students also need to creatively tell others about their findings, and can choose to write a blog post for this site as a way to communicate their findings. For more information on this assignment, just use the contact form on this website to get in touch and we can share more details.
By Julia Stegmaier
Imagine identical twins. You probably think of two individuals with almost identical appearances. In various TV shows, books, and articles, identical twins appear as the most alike individuals on the planet. Perhaps you are wondering if identical twins, also known as monozygotic twins, are inherently the same because of sharing genetics. Thomas Bouchard’s famous study, The Minnesota Study of Twins Reared Apart, plays a significant role in research findings on general intelligence, mental abilities, and personality characteristics in identical twins growing up in different environments. But how similar are monozygotic twins really; do they differ in personality characteristics and IQ, or are they the same?
One of the most notable twin studies was by Thomas Bouchard, a professor of the psychology department at the University of Minnesota. This study involved 137 pairs of twins studied within 20 years and is part of the Minnesota Twin Study, involving various twin studies since 1979. Besides, the study investigated identical and fraternal twins reared apart.
In particular, the also famous Jim Twins, named James Arthur Springer and James Edward Lewis, were one of the assessed twin pairs in Bouchard’s study. At only four weeks old, the Springer family adopted one twin, and the Lewis family adopted the other twin, not knowing about the existence of each other. A reunion at the age of 39 led the reared-apart twins to a relationship described as closer than best friends (Segal, 2012). Initially, Bouchard intended to focus on factors involving general intelligence and intellectual ability. The Jims were the first ever separated twin pair to assess various psychological and psychophysiological procedures including life stress and sexual life history assessment.
Now, let’s take a closer look at whether growing apart affects the IQ and personality of the Jim twins. But what does IQ even mean? The American Psychological Association, APA, defines IQ as “a standard measure of an individual’s intelligence level based on psychological tests.” Along with the results of Bouchard’s study, the reared-apart twins showed only a difference of nine points in IQ scores in total. By this means, genetics strongly impacts the variance in intelligence. Yet, do they differ in terms of personality? The study findings indicate that identical twins seem similar in their behaviors and interest; this includes identical twins growing up together and apart from each other. Even more impressively, the Jim twins share far more similarities in personality. Both share interests in mechanical drawing, block lettering, and carpentry (Segal, 2012); in addition, the reared-apart twins seem to share similar behavior, and their gestures of the twins seem just about identical. Essentially, the study findings of Bouchard indicated no difference in general intelligence and personality characteristics in monozygotic twins reared together or reared apart. Overall, the environment that identical twins grow up in does not impact general intelligence in identical twins.
A recently conducted twin study revealed another story of a twin pair reared apart. The female twins grew up separately: one in the United States and the other in South Korea. Interestingly, the intellectual abilities of these twins show a big gap, as they are 16 points apart. While these findings differ from the Jims, their personality characteristics were revealed to be immensely similar. Although each twin grew up in a different environment, the female twins’ personalities appear purposeful, well-organized, dutiful, and achievement-striving (Segal, 2022).
The exceeding resemblance of monozygotic twin pairs raised in different environments is impressive. Without James Springer and James Lewis being immensely alike, data from over 100 pairs of twins may never have come to light. Along with this, the study by Bouchard may be considered a cornerstone in the history of twin studies. Above all, studying identical and fraternal twins enable us to understand the significant role of genetics and environmental factors in human development.
Twins share far more than a birthday. Indeed, identical twins reared together and apart are excessively alike in their appearance in personality and somewhat similar in mental abilities. It is common to think that identical twins have the same interests, behaviors, and intelligence. Nonetheless, even they differ in some ways. Next time you see identical twins, you may want to observe them more closely. At first glance, do you view them as nearly identical?
References
American Psychological Association. Apa Dictionary of Psychology. American Psychological Association. https://dictionary.apa.org/iq
Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N. L., Tellegen, A. (1990). Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart. Science (American Association for the Advancement of Science), 250(4978), 223. https://doi.org/10.1126/science.2218526
Chen, E. (1979, December 9). Twin Reared Apart: A Living Lab. New York Times. https://www.proquest.com/historical-newspapers/twins-reared-apart-living-lab/docview/123888694/se-2
Segal, N. L. (2012). Born together-reared apart: the landmark Minnesota twin study. Harvard University Press.
Segal, N. L., Hur, Y. M. (2022). Personality traits, mental abilities and other individual differences: Monozygotic female twins raised apart in South Korea and the United States. Personality and Individual Differences, 194, 111643. https://doi.org/10.1016/j.paid.2022.111643