“Twins studies are the only real way of doing natural experiments in humans. By studying twins, you can learn a great deal about what makes us tick, what makes us different, and particularly the roles of nature versus nurture that you just can’t get any other way”

_ Tim Spector(professor of genetic epidemiology at King College, London and director of the TwinsUK Registry)

Twins are considered natural living laboratories in psychology. Identical or monozygotic twins share all of their genes, while fraternal or Dizygotic twins share about 50 percent, the same as any other sibling. So, if a trait shows higher similarity within identical twins than fraternal twins, it would imply that genetic factors are partly responsible for one’s development. This interesting relationship within twins allow researchers to examine the genetic and environmental influences over physical and mental health, traits or characteristics which could possibly be anything, height, personality, or mental diseases.

Moreover, twin studies are very effective in showing the outcome of unique environment, such as birth defect and head injury, in one twin while the other remains in a healthy condition. Therefore, studies of twins have been a key tool in untangling controveries over genetic and environmental influences, disclosing the roles of envirionment factors ,shared or unique, and genetic factors in shaping one’s behavior.

For this reason, twin research is currently being conducted in more than a hundred countries. They are aimed to study the genetic and environmental causes of age-related complex traits and diseases, and ultimately, to enhance the understanding of human and our health. Let’s look into the way these researches are conducted and a few famous results.

Methods of Twin Studies

- Co-Twin Control

Among many methods in twin research, there is method called co-twin control which is used in research with identical twins. This is also divided into two cases: natural and experimental.

Natural co-twin control takes place when identical twins show differences in appearance, health, or behavior which are not the outcomes of any kind of intervention. It includes identical twins who show the opposite in characteristics such as hand preference, fingerprint patterns, or direction of hair whorl.

Experimental interventions happen when a researcher takes certain measures, sepcific treatment or training programs, while providing the other twin with different experience or none at all. There is an exmple from a classic twin study: one twin was given training in stair climbing and cube manipulation while the other was not. After some time, the twins’ abilities were measured and compared, but were found to be not much different, meaning that training was not responsible for the abilities.

- ACE model + SEM(Structural Equation Modeling)

The most widely used method to compare the influence of genes and environment is “ACE model”. In this model, the sum total of differences between individuals is splited into tree broad variations”

1) A, additive genetic effects: factors that identical twins share one-hundred percent and fraternal twins share fifty percent. This means genes.

2) C, common or shared environmental effects: factors shrared one-hundred percent between twins regardless of whether they are identical or fraternal.

3) E, unique or non-shared environmental effects: factors unique to each individual no matter the identical or fraternal.

Twin reaserchers estimate the amount of individual differences attributed to each of these factors using a method called Structural Equation Modeling(SEM). It is commonly used in social sciences owing to its ability to impute relationships between latent(or unobserved) variables from observable ones. The latent variables are difined by one or more observed variables. For example, human intelligence cannot be directly measured. So instead, psychologists develop a hypothesis of intelligence and create measurement instruments consisted of questions designed to measure intelligence according to the hypothesis. Then, SEM would be sued to test their hypothsis using the results of intelligence test.

This model is extremely useful in measuring and identifying genes and environment; however, the results only show the effects in general and does not identify the specific genes and environments that make differences.

Results of Several Researches

1)Genes and IQ

In 2003, Eric Turkheimer, a psychology professor at the Univ. of Virginia, took a new veiw point at the former researches of I.Q., and noticed that most results are from twins born in middle-class backgrounds. He got interested whether the result would be different if twins from poorer families were measured. As the result, he found that the I.Q.’s of identical twins varied just as much as that of fraternal twins, which were the opposite of the prior researches that showed similar I.Q. within a pair of identical twins. This research revealed that the impact of growing up poor can overwhelm a child’s inborn intellectual gifts.

2) “Gay Gene”

In psychology, there had been numerous approches to elucidate the importance of genes in sexual orientation until the research led by Niklas Långström, a psychiatrist, in 2008. He drew upon the twin data from Sweish Twin Registry to investigate whether genes and environment determine a person’s gender identity. The result found that genes accounted for only thirty-five percent of the differences between identical and fraternal gay men and even less in lesbian women, around eighteen percent, meaning that the percentage was under fifty percent for both sex. This study indicates that a person’s sexual orientation is not an inborn nature, but a complex interplay of genetic and environmental factors. But still, this result cannot be a hundred percent reliable, since only twelve percent of males from the Swedish Registry were included in the study. (This is the fallacy of social sciences not leaving out psychology.)

3) Twins reared apart

One of the most shocking result in the history of psychology is here in the field of twin studies. In 1979, Thomas Bouchard conducted a fascinating twin study: study of twins reared apart. Bouchard looked at identical and fraternal twins separated since infanvy and found that identical twins who had different upbringings often had remarkably similar traits and nearly same choices! Bouchard came across a pair who had been separated from birth and reunited at the age 39. Bouchar wrote: “The twins were found to have married women named Linda divorced, and married the second time to women named Betty. One named his son Jame s Allan, the other name his son James Alan, and both named their pet dog, Toy.” Of course, there are criticism that this sole evidence cannot point to genetic determinis; however it shows that we are not born into this world as random beings or balnk slates! Genes definitely takes its parts, and it could be bigger that we had ever expected. As mentioned above our traits are the results of interplay of genes and environment.