One century of research on twin-singleton differences in intelligence: A preregistered meta-analysis and systematic review

Twin-singleton differences in IQ are of concern for many groups, starting with parents of multiples and educators, up to behavioral geneticists (who depend on twin-singleton equivalence for generalizing twin study results to the general population) and cognitive epidemiologists (who investigate IQ associations with morbidity and mortality). It is thus unsurprising that twin-singleton differences in IQ have been investigated since the advent of twin studies and IQ test batteries. We significantly update and expand the sole meta-analysis on this topic. Extensive literature search strategies identified 495 eligible effects from 117 studies from 24 countries (totaling 6,500,000+ singletons and 145,000+ twin individuals), published 1924-2019. Multilevel meta-analytic modelling showed that twins scored about 4 IQ points lower than singletons: a non-trivial effect, taking into consideration tail ratios and liability-threshold models. Further, the effect was temporally stable, robust with respect to publication bias, generalizable to a surprising scope (across sex, geography, premature birth status, and assisted reproductive technologies), whilst systematic sources accounting for observed cross-study effect heterogeneity remained mainly elusive. The conundrum of this group effect in IQ seems not satisfactorily solvable with the available evidence. Specifically, data from adults and less developed countries remain a desideratum.