We are the same… just different.
Have you ever after woken up in the morning, looked in a mirror and though, that does not look like me at all? Obviously this is because not many people look great in the morning, however it might also be that you, in your daze of tiredness mistook your identical twin on the other side of the doorway for being you.
Of course the chance of having an indentical twin is extremely small, in fact it is about 3/1000, however if you happen to be either an identical or fraternal twin you are very important for scientific research into genetics.
The approach of twin studies, in psychology, is generally used to show how much your personality and behaviour can be attributed to either nature or nurture, by comparing comparing fraternal twins to identical ones, and then compare differences between them, either in shared environment, or when twins have been adopted, and see if they still share some features in personally or sometimes even personal history. For example will you see, in adopted twins, that when they grow old they will have, to some extend, similar personalities and also interests and hobbies, sometimes even same view on attractiveness. (Olson et al., 2001). The results show how big a correlation there is between genetic interaction and environmental factors.
It seems to be a quite simple way to study things, i.e., if two identical twins significantly more share a trait than fraternal twins, then the cause of this trait ought to be caused by genetic factors, however think about this, in psychology we have accept results if they generalize for 95 % of the population, however dealing with twins, that is a population even below that number, they might all be extreme cases for what was measured, and therefore maybe because of their shared genes, they share a bias towards certains measures that normal people do not have.
Another limit to consider in twin studies, they can only measure correlation, nowadays most scientists have accepted a soft deterministic approach, that we have free will to some extent, but that our genetic make up will limit us in some way. So the studies can only show that ones genes, for a bigger or smaller part affects ones personality. And this is the biggest limitation to twin studies, and with the current technology I just don’t see any way this approach can move any further, than finding more correlations. But that is just what I think, you might have a different opinion.
Nirapsy 
You have considered a number of disadvantages to the use of twin studies and I can agree with you that twins aren’t representative of the whole population but twin studies are very important. Twin studies as you have said can show us an interation between genes and the environment. From these studies we can investigate the concordance rates of diseases, with twins sharing such a large amount of genes and with MZ twins sharing all genes, we can determine the likelihood of susceptability to disease and illness and whether or not the environment has a part to play. These studies can be used to tell us how genes work, what certain genes do what and how heritable diseases are. Twin studies also allow us to investigate the impact of the environment on genes in an ethical way, we cannot breed and raise twin humans to look at interactions, they are pre-existing groups that can be studied.
I disagree with you; I believe twin studies will continue to posses’ value in the future, as research techniques continue to improve. The use of meta studies has allowed researchers to group huge amounts of data across several twin studies to produce robust heritability coefficients.
An interesting piece of research to consider when evaluating twin studies is that of Kendler et al (1993). They studied monozygotic twins whom had been raised as non-identical twins mistakenly by their parents. They demonstrated that these twins’ traits remained as concordant as twins raised by parents that treated them as identical. Therefore, even in an attempt to raise two individuals from identical twins, concordance rates were unaffected. This suggests it possesses high validity as a heritability measure.
Of course, i cant possibly argue that limitations exist concerning the use of twin studies. However, i do also feel that the limitations don’t completely invalidate the usefulness of twin studies.” For traits that are substantially influenced by heredity, the approximately two-fold difference in genetic similarity between the two types of twins should outweigh any complications” (John Hewitt, PhD, director of the Institute for Behavioral Genetics at the University of Colorado at Boulder)
Although its difficult to improve of twin studies considering we cant reeeally (well, ethically) increase the twin population there are some ways in which the designs and methods are evolving and improving. Many studies have now extended their range, looking into families with all sorts of variations. Including case studies, longitudinal studies and extended families. Some are even including the spouses of twins and comparing their traits etc.going beyond the classical twin study design.
some of the advancements of twin studies and designs are discussed in this paper and is actually quite interesting : http://www.biostat.sdu.dk/courses/f11/TwinAnalysis/papers/Twin%20Studies/boomsma2002.pdf
oooh sorry…. i obviously meant.. cant argue that limitations DONT Exist. they very much do !
An interesting post being a fraternal quadruplet (take that twins!) myself, it’s nice to know I’m valuable to scientific studies 😉
I agree with your comments that the chances of find twins are rare and that the results can’t be generalised to the whole population, but then again this applies to most studies. However I disagree with you that twin studies can’t move further than showing correlations. They allow us to see a very unique set of situations, through identical and fraternal twins and the different environmental up-brings that may happen in a person development are still important findings. Even if the genetic studies may have run their course.
Personally, I think twin studies are extremely useful in studying the heritability of genes/traits! Obviously, they have their drawbacks – they’re such a small percentage of the population that we can’t realistically generalise to a whole population. Its also possible that the environments of identical twins are going to be much similar to those of regular siblings because they are the same age, they may go to the same school, have the same friends/influences etc – it can be difficult to separate the genetic from the environmental influences of identical twins. However, I think that so long as twin studies aren’t taken completely at face value, and the role of environmental influences isn’t completely discounted due to the high concordance rates (presumably due to the identical genes of MZ twins), they are still really useful in research. Baker & Daniels’ study into genetic influences in twins did just this – paid attention to the environmental influences as well as the apparent genetic ones (http://psycnet.apa.org/journals/psp/58/1/103/)!
I found a great article in the national geographic magazine regarding identical and fraternal twins and the argument of nature vs nurture.
http://ngm.nationalgeographic.com/2012/01/twins/miller-text
I half agree with you saying that they can only measure correlation, as we can only say that genes PREDISPOSE us and that the envi influences the outcome. But a lot of technological measures have been used to try and distangle the differences. To confuse things even more this article talks about a third contribution, epigenitcs. They can even sometimes serve as a bridge between the environment and our genes. epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence – hence the name epi- (Greek: επί- over, above, outer) -genetics.
Great subject to choose. Being a fraternal triplet, I like to think that we offer some serious research possibilities. I will agree that twins are not representative of the whole population, but then is any research? Twin studies are invaluable, they are able to identify whether environmental issues play a part in diseases or whether its gene based and the concordance rates of these diseases.
Sir Francis Galton began to study how genetics influence intelligence over a century ago. Galton theorized that children inherit their intelligence from their parents. Thus, he believed that genes were accountable for intelligence. Galton turned to twin studies to test his theory.
The Minnesota twin study, observed a set of identical twins that were raised in separate locations. The twins did not meet each other until they were almost 40 years old, The twins showed that they shared many similar characteristics. The only explanation for the similarities was that the twins’ genes had strongly guided their development. The research concluded that, on multiple measures of personality and temperament, occupational and leisure-time interests and social attitudes, identical twins reared apart are about as similar as identical twins reared together.
Twin studies like the Minnesota Twins are valuable research in the nature vs nurture debate.
http://www.nurture-or-nature.com/articles/twin-and-adoption-studies/index.php