Saturday, October 16, 2021

Comments by cmd

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  • “However, there is also some evidence that MZ twins are treated more similarly by their parents and have more frequent contact as adults than DZ twin pairs.(13)”

    This is a rather blanket reference, any particular data from this book in regards to sharing similar environments? Jay Joseph mentions that a 1967 schizophrenia twin study by Kringlen showed that over 90% of MZ experience “identity confusion” during childhood (only 10% for DZ), 72% of MZ were “brought up as a unit” vs 19% of DZ, and 65% of MZ had an “extremely strong” level of closeness vs only 19% for DZ. Jay Joseph also mentions that “I’ve never seen these results discussed by any twin researcher other than Kringlen.” (1)

    “Note: there are also factors that can have the opposite
    effect and increase variability between MZ twins. One example is when MZ twin pairs are forced to attend different classes at school, while DZ twins are allowed to remain in the same class. This could lead to an underestimation of the genetic effect.”

    The magnitude of effect this example is paltry compared to the marked effects of similarity noted in the Kringlen reference.

    “How do we detect this effect?If parental treatment is more similar for MZ twins, than DZ twins who are mislabelled as MZ twins should be more alike than correctly labelled DZ twins and conversely, MZ twins mislabelled as DZ should be less alike than correctly labelled MZ twins. Little or no effect of mislabelling was found.”

    This confuses me, when would we “mislabel” MZ twins? It’s pretty apparent they’re identical. Have you read the reference so that you can expound on the above statement?

    “The effect of degree of contact among twins showed that more frequent contact does not lead to behavioural similarity in same-sex DZ or MZ twins. While in some cases MZ twins in frequent contact were more similar than those with less contact, these correlations tended to be small (16). Another argument in defence of the equal environments assumption is the fact that studies of MZ twins reared apart have provided correlations for personality variables that are almost the same as those for MZ twins reared together (17)”

    Jay Joseph has thoroughly gone over the many limitations of MZ twins reared apart research and has an extensive critique of the Bouchard study cited above at reference (2)

    1) Joseph, J. Twin Studies in Psychiatry and Psychology: Science or Pseudoscience. 2002.

    2) Joseph, J. Separated Twins and the Genetics of Personality Differences: A Critique.
    American Journal of Psychology, 114, 1-30. 2001.