“…We then re-evaluate a set of hypotheses that have been put forward to explain variation in national IQ as a case study to provide guidance for future studies. Note that while the global variation in mean national IQ has received considerable recent attention, it remains debateable whether variation in national IQ is a strict reflection of variation in underlying cognitive abilities that they are proposed to measure, since their psychometric properties may also vary across space (Wicherts, Dolan, Carlson, & van der Maas, 2010) and time (Wicherts, et al, 2004). For example, recent work has indicated that IQ score may vary with individual motivation, and that this simple phenomenon may confound relationships between individual IQ and late-life outcomes (Duckworth, Quinn, Lynam, Loeber, & Stouthamer-Loeber, in press).…”