Why don’t you look exactly alike?

If you’re identical twins, why don’t you look exactly alike?

This question is invariably posed by the ignorant with a note of suspicious challenge and more than a hint of smarm, as if you know, maybe Lillith and I are just strangers who happen to look, sound, and think quite a bit alike and are just passing ourselves off as identical twins for some nefarious reason.

Our response to this query is usually, “When we were children, we looked a lot more alike. We’ve grown to look different.”  These gradual appearance changes go deeper than the skin, though, actually reaching a genetic level.   SRI International provides an explanation:

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off.  This is called epigenetic modification. A study of 80 pairs of human twins ranging from age 3 to 74 showed that the youngest twins have relatively few epigenetic differences.  The number of epigenetic differences between identical twins increases with age. Fifty-year-old twins have over three times the epigenetic differences of three-year-old twins.  (“More Fun Twin Facts.” SRI International Northern California Twin Registry Newsletter, Apr. 2008: 2)

You can read more about epigenetic changes at Explaining Differences in Twins at Twin Data Highlight Genetic Changes.