Twin studies are widely used in evolutionary psychology as evidence of genetic determination in behaviour. Identical twins share 100% of their DNA. They look similar, and arise when one egg splits to give two eggs with the same DNA (monozygotic). Fraternal twins look less alike, and may also be of different sex. Although they develop in the same womb, they arose from two separate eggs so share just 50% of their DNA (dizygotic). Identical twins, even when reared apart, can have an uncanny similarity in behaviour, more so than for similarly treated fraternal twins. Surely proof that genes rule. But for Dr Beetle, there is another way to understand the results. The similarities are more likely to be due to learning and shared environmental experiences. There are four major assumptions that undermine the evolutionary psychology approach to twin studies:
Assumption of learning absence in womb
One of the common mistakes is to assume that birth marks the beginning of learning. If assumed, then any similarities between identical twins separated at birth will be due entirely to genetics. However, there is ample evidence to show that the foetus learns, probably within a few months of being conceived. Sounds, smells and tastes are amongst the first environmental cues learnt, ahead of vision. Sometimes the foetus will 'jump' when an outside loud noise is heard, and some parents know how to make their unborn baby kick on Que. The foetus may also learn the tone or mood of the womb and mother, and so probably pick up on a level of anxiety, health, hunger, depression, intoxication, rest, comfort and stress. As one example, the foetus of a heroin addict will emerge an addict themselves, and needs careful doctoring to take them through withdrawal. Similarly, if a pregnant woman included garlic or vanilla in her diet, the baby will choose similarly laced milk over normal milk when born, see Huotilainen. The foetus also learns to recognise the mother's voice, and will hone in on that sound as a baby. Listening to music (the Mozart Effect) while pregnant appears to improve the mental stimulation of a foetus, and improves musical ability. Anti-abortion followers regularly come up with examples of attentiveness and alertness in the foetus. For twins, the womb environment will provide similar foundations for their future development, even if separated at birth. It is interesting that separated twins, not knowing they were twins but reunited as adults, often say that they always felt something was missing, or that being a twin feels right. I don't believe in a psychic connection, so this sensation suggests to me some memory of the womb environment. Granted nevertheless, if this were the sole factor involved in producing the similarity found in twins, then one would expect that fraternal twins would be as similar as identical twins when both are separated at birth. That identical twins are more similar suggests additional influences from learning.
Equal environment assumption
Another assumption is that identical and fraternal twins will experience the same morals, expectations and experiences when raised together. If identical twins then turn out to be more similar than fraternal twins, then their same genes are the reason. However, it should be obvious that it is wrong to make such an assumption. People tend to treat identical twins more similarly than fraternal twins, simply because they look alike. They will have a greater expectation of similarity 'imposed' on them by others. Identical twins will also realise their own physical similarity, and may mimic each other consciously or subconsciously. They could expect to be close friends for life. After all, it can be a great party trick to switch places and fool others, and even trick potential boy or girl friends. Identical twins would naturally explore the joys of similarity more than fraternal twins, because any mirror will remind them of the closeness of their relationship. Some twins even learn a kind of in-house twin-talk or idioglossia that few others can fully comprehend. Another cause of similarity in identical twins is that they will always be the same sex, while fraternal twins may be different. Without understanding the greater similarity of social influences acting on identical rather than fraternal twins, the influence of genes has probably been overstated in past twin studies, see the writings of Jay Joseph (and his Developmental Review 2000, vol 20, pp 539-593 is a good read), and Horwitz, A. V., Videon, T. M., Schmitz, M., & Davis, D. (2003). Rethinking the Relationship Between Twins and Environments: Possible Social Sources for Presumed Genetic Influences in Twin Research. Journal of Health and Social Behavior, 44(2), 111-129.
Assumption that separation means separation
Another assumption is that separation at birth means no continued sharing of environmental conditions that could then cause an overlap in learning. But identical twins separated at birth are often placed with a relative or someone approved by the mother, who is like-minded or shares the same belief systems. Assignment to adoptive parents is not random. Also, any line of communication between identical twins through relatives or pen pal letters, could introduce a merging of learning in ideas and mannerisms. Given any chance, separated twins are very likely to look for the things they do similarly, and then dwell on them. Developing bonds, and finding soul mates who you can really talk to, is a prize for many humans. Twins may even start to mimic each other's mannerisms soon after meeting. If you have any doubt about the appeal of adjusting personalities towards a home base, look at the number of people who are convinced that they share similar personalities, simply because of their star sign. Self adjustment to star sign 'traits' is assisted by the many magazines and horoscopes that say what is normal temperament for their star sign. The power of self-adjustment, perpetuation and delusion as a means for ingraining rules and trends into the mind is mightily underestimated amongst humans. Even an unsuspecting researcher who came upon the rare examples of true separation could unintentionally put examples of similarity into the minds of the separated twins. I wonder how many of the separated twin studies concluding inherited behaviour can really claim to be free of this influence, and how much is just smoke and mirrors. Several researchers also doubt that these influences have been properly negated. The combination of true separation, exclusion from family contact and belief systems, and researcher independence is rare.
Assumption that similarity comes from pre-wired brains
There would still seem to be a greater degree of similarity between identical twins than can be explained so far. The final assumption is that similarity of behaviour arises from pre-wired brains and instincts, rather than induced learning from the surround of similar structures (same eyes, height, voice etc). Sure, similar behaviours could be produced either way, but it is important to recognise which mechanism is operating, as then we could understand how the mind works, and realise who is in the driver's seat. Is it the genes and their pre-wired instincts, or the process of learning where with understanding you could then develop greater self control and notice more potential? Separated twins are likely to need glasses at similar ages as they have similar eyes. If born with the same speed of reflexes, stature and musculature, it is likely they will perform similarly at sport. If both are tall, they may both take up basketball or volleyball. They might go bald at the same time, and require the same comb-over. Identical women with blonde hair might attract the same trophy seeking men. If born with the same taste buds, twins are likely to grow to like the same foods and tastes. If the capillaries under their skin are particularly flush and thick at the back of the calf, perhaps they will both enter water backwards rather than forwards. Similarly, blood capillary distribution may influence whether they are both more or less attractive to mozzie bites. Another factor that I think is important in behavioural development is the foibles of the internal clock, and the diversity of the feedback loop that reaches from the cortex to the pleasure centre. All of these are physical features from which learning could occur. The main physical features are coded by DNA, but I do not agree that neural pathways are already fixed in the brain, or that DNA codes for instinct and behaviour occur, to keep you forever bound and troubled. Humans should recognise the relationship required with their environment and body, as then they could enhance the ability to learn and mentally receive new things. Believing in inherited codes downgrades the importance to development of getting it right and coming to terms with your surroundings, rather than thinking you have to ride buckshot on your high horse of unchangeable selfish DNA.
In summary, the similarity between identical twins separated at birth can be easily explained by the mechanisms of foetal learning, a desire for bonding and soul mating (leads to mimicry), and predictable learning induced by the similarity of environment and bodily structure. A simple test of which hypothesis is accurate is, if inherited, then show me the inherited instinct or mind module in a baby, rather than claiming inheritance after those instincts are learnt and solidified in children and adults. None have been revealed, further demonstrating that evolutionary psychology has no foundation, and is still based upon guess work and innuendo. In comparison, the mechanism of learning and the ability of uncommitted neurones to align and consolidate into certain patterns based upon the stimulation they receive (experience and interaction) have been amply demonstrated (more beetle). (Article posted October 2004).