h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I was wondering, in the case of female identical twins, won't there usually be some 'genetic' differences as a result of variation in the pattern of X-chromosome expression in the brain.
(Similarly, though at a lesser level, the possibility of limited chimerism even in males could result in occasional differences along similar lines.)

It may well be that differences are often below the noise level involved in the measurements, but I imagine that if there are genes on the X chromosome that seriously affect brain structure or function, (and there is significant variation in those genes within the population) there *could* be female identical twins with quite different brain layouts.