The shared-family environmental hypothesis for sex differences in cognitive decline: support from an Italian general population twin study

In our twin study, we had the unique opportunity to verify the above aetiological mechanism of decaying cognition, for the first time in the Italian general (i.e., non-clinical) population. If the mechanism is true, then in a genetically-informed study of MZ and DZ twins, one should be able to detect a significant estimate of the shared environmental component for the targeted cognitive measure; furthermore, a certain degree of underestimation of genetic effects should be expected. This is exactly what we found for the total SAGE score in females: no evidence of heritability emerged, and one-quarter of variance was suggested to originate from the family background, with the lower bound of the 95% CI for the estimate being well above 0, even with a relatively low number of female pairs (82 MZ, 61 DZ).