I will be presenting results from my current project at the 15th Conference on Personality and Psychological Assessment on The conundrum of twin-singleton differences in intelligence - A centenary (1924-2018), preregistered meta-analysis. You can find the slides on my OSF project.
It will be part of the symposium Meta-analyses, meta-meta-analyses, and meta-research in individual differences research, see a complete description in my previous post!
Twin-singleton differences in IQ would be 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 (Merriman, 1924). We significantly update and expand the sole meta-analysis on this topic (Voracek & Haubner, 2008). Extensive literature search strategies identified 453 eligible effects from 108 studies from 25 countries (totalling 6,100,000+ singletons and 184,000+ twin individuals), published 1924-2018. Multilevel meta-analytic modelling showed twins scored about 4 IQ points lower than singletons: a non-trivial effect, when considering tail ratios and liability-threshold models. Further, the effect was temporally stable, robust with respect to publication bias, generalized to a surprising scope (across sex, geography, measuring instruments, premature birth status, and assisted reproductive technologies), whilst systematic sources accounting for observed cross-study effect heterogeneity remained 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.