Identical Twins

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One intuitive vantage point from which to see a number of the difficulties confronting the DNA model - and also the encompassing materialist vision - is with monozygotic twins.  These so called identical twins in fact present a number of basic mysteries.  The cause of their origin - the initial split or division of a single cell zygote - is a mystery.  So is its occurrence only in some species.  Further, similar appearances and biological-presumptions aside, the empirical realities of the resulting twins constitute a significant challenge to the sacred DNA "created us body and mind" logic that motivated the Human Genome efforts and associated excitement.

Although DNA replicas, such twins whether they were raised together or separately, have been observed on average to be more different than alike personality-wise.  Thus these clones can closely share the same environment or inhabit separate ones (with whatever differences in epigenetic implications) and still they appear to have comparably different personalities.  Notice this environment (or nurture) minimizing finding is also consistent with the conclusions of adoption studies.  With some personal exposure to identical twins and/or after reading relevant study data, it should not come as a surprise that one conjoined (attached) monozygotic twin commented that "[w]e are two completely separate individuals who are stuck to each other.  We have different world views, we have different lifestyles, we think very differently about issues" [Harris J., p.1].  This mystery motivated Judith R. Harris' No Two Alike and was discussed at some length in Steven Pinker's The Blank Slate [Pinker 2002].

Yet in the case in which identical twins were separated at birth, they can still share remarkably specific behavioral tendencies as mentioned in an earlier quote from Pinker.  From the small and questionable stuff (look enough and any pair of humans might find they share some little preferences) to the big and life-defining stuff - like becoming very dedicated volunteer firemen [Segal, p.14].  Such phenomena have been used as strong evidence for the life-defining import of DNA, but set against the seemingly innate personality differences they could be just another mystery.

Large health differences between monozygotic twins were described in a 2006 New York Times article by Gina Kolata [Kolata 2006].  That article opened by describing a healthy and active 92 year old and her identical twin.  The latter "is incontinent, she has had a hip replacement, and she has a degenerative disorder that destroyed most of her vision ... [and] has dementia".  Yet the sisters have the same DNA, grew up together, and lived in the same place.  They also had markedly different personalities and ambitions.

The centerpiece of Kolata's article was a description of a large study comparing the variations in longevities found amongst identical and same-sex fraternal (or dizygotic) twins.  Since the former share all of their variable (and potentially individual-differentiating) DNA and the latter only half, comparisons like this are standard practice for determining the relative import of DNA, as in behavioral genetics.  Continuing, together the study's twin populations totaled 10,251 pairs.  The study found that the identical twins died only a little closer together than the fraternal twins and, specifically, the deaths of the identical twins averaged "more than 10 years apart".  Consistent with this, one of the twin study's authors commented "[h]ow tall your parents are compared to the average height explains 80 to 90 percent of how tall you are compared to the average person [but] only 3 percent of how long you live compared to the average person can be explained by how long your parents lived".  The relevant Nature Review Genetics paper gave the inferred genetic component as covering - "about a quarter of the variance in adult lifespan" - and pointed out that somewhat surprisingly, "the genetic influences on lifespan are minimal before the age of 60 and only increase after that age" [Christensen et al].  As representative of the intellectual momentum associated with DNA (and science), though, Kolata's article had been titled "Live Long? Die Young? Answer Isn't Just Found in Genes".

Kolata's article also mentioned that "randomness" had been found amongst the longevity of genetically identical lab animals.  Additionally, Dr. Robert Hoover of the National Cancer Institute was quoted from an editorial on the cancer connection, "there is a low absolute probability that a cancer will develop in a person whose identical twin - a person with an identical genome and many similar exposures - has the same type of cancer."

It is noteworthy, though, that these very significant and unexpected findings (and article) seem to have been minimally integrated into relevant scientific communication.  This is possibly common for results that challenge materialism.  For example, four years later in a Scientific American review article on the lack of findings in personal genomics the word "twin" never appeared [Hall].  If you are trying to find DNA which is significant to the different health outcomes of individuals who are broadly separated by environment and their variable portions of DNA, wouldn't it be good idea to gauge these efforts - and more generally inform others - on the large health differences found between individuals who are minimally separated by environment and share their DNA specifications?

Another monozygotic conundrum is the remarkable bond that tends to exist between them.  As pointed out by Steven Pinker, "when separated at birth and reunited as adults, ... say they feel like they have known each other all their lives" [Pinker 2002, p.47].  In one of my childhood neighborhoods I can't even remember the local twins being apart.  Given that siblicide is common in nature does this really make sense for siblings [Tenneson]?

Finally, another monozygotic-mystery can be found with regards to male exclusive homosexuality. This behavioral tendency appears to be established by birth and of course is a challenge to evolutionary reasoning.  The DNA contributions can not be big, though, since when one monozygotic twin is gay then the other twin is gay in only about 20 to 30 percent of cases (against a backdrop overall gay frequency of 2 to 4 percent).  Additionally, it appears that the likelihood of a male having a homosexual orientation increases by about 30 percent for each older brother preceding him.  As mentioned earlier this information was found in Francis S. Collins' confident DNA book, The Language of Life [Collins, pp.204-205].  Thus science's gay explanation has to identify a loose DNA contribution, find the means to ramp up the gay likelihood by 30 percent per older brother (presumed to be caused by the mother's previous male-pregnancy experiences), and also perhaps identify some other subtle prenatal environmental influences to wedge apart the twins' orientations.  Does this seem "bluntly deterministic" or consistent with the earlier cited programmatic assessment of Sam Harris?  In fact these findings, like others not related to appearance, are contrary to DNA-based expectations and suggestive of a broadly-based mystery.