From the study: "It is important to consider whether specific skills—those described as “malleable but peripheral” by Bailey et al. (2017, p. 15)—or general abilities—such as the general g factor of intelligence—have been improved (Jensen, 1989; Protzko, 2016). The vast majority of the studies in our meta-analysis considered specific tests and not a latent g factor, so we could not reliably address this question."