Researchers have a long interest in twins. About 150 years ago, Sir Francis Galton, Charles Darwin’s cousin, investigated the distribution of behavior and certain abilities in and between families, and especially between twins, and is therefore considered to be the founder of twin studies. To this day, attempts are made to answer the question “object or environment?” using twinning research. In other words, whether certain traits are determined by genes, the environment, or even both.
This is why identical twins are of particular interest to research. Environmental influences on them can be studied particularly well because of their almost identical genetic information. When evaluating twin studies, however, it should be noted that even identical twins are not 100 percent, but only 99.99 percent genetically identical, as is known today.
How does twin research work?
However, it is not only identical twins that are of interest to twin research. This also applies to fraternal twins, as they do not match their genetic information as much as identical twins, but are also born at the same time and therefore are subject to the same or at least very similar environmental influences from conception. Therefore, they serve as a comparative group in classic twin studies.
Then scientists study the so-called compatibility, the similarity of certain features of monozygotic twins compared to fraternal twins. From this they can conclude: If monozygotic twins are more similar in this trait than fraternal twins, it is a sign of genetic influence. On the other hand, if monozygotic twins differ in strikingly certain characteristics, this may indicate an influence of environmental factors. Of particular interest are identical twins that were separated at birth. Here, twin researchers check how similar twins are despite their separation and what makes them different.
And the insights?
Gemini researchers have studied every human trait imaginable in this way for years. From height, weight, favorite food or athletics to political beliefs, religiosity, sexual orientation and choice of a partner, to predisposition to mental illness, cancer or aggressiveness.
The question of the heredity of intelligence has long been very popular. With the help of a study of twins, researchers concluded that up to 70 percent of them are inherited. This means that 70 percent of the deviations from the population mean intelligence can be attributed to genetic factors. However, this value only refers to the average and not to the individual intelligence of a person.
Conversely, twin researchers have established that the social environment and upbringing are more important than genes when it comes to personality development. Traits such as timidity and openness are thought to be only about 30 percent genetic, and the genetic influence on political or moral attitudes is probably very small.
Twin researchers are also very interested in life expectancy and disease development. Many different diseases, such as cancer or chronic inflammatory diseases of the gut and nerves, are currently under investigation for their genetic causes. However, thanks to highly developed methods, the DNA itself can now be studied so thoroughly that scientists can discover minimal differences between identical twins and infer disease-causing genes.
In addition, researchers gained important insight into epigenetics through the study of twins, environmental factors that influence gene activity and affect health individually and independently of actual DNA.
Overall, twin researchers are concluding today that both aspects, genetics and environment, always play a role, albeit in different proportions. However, the exact interplay of these factors is very complex and in many cases only partially explained.