Answers are grounded in this paper and cite their sources.
Twins are not a homogeneous group. According to zygosity and chorionicity essential differences exist. The lower sex proportion at birth is due to the monozygotic twins, especially the monoamnionic variety. X-inactivation patterns in monozygotic twin girls are totally symmetrical in monoamnionic pairs, almost symmetrical in monochorionic diamnionic pairs and can be very asymmetrical in the dichorionic variety.
No takes yet. Share an insight, caveat, or question.
Derom et al. (2007) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: