Different Measure of Intelligence Peak at Different Ages

One potential concern with cross-sectional data is that it may be subject to cohort effects. Our findings in Experiment 2 are consistent with the possibility that people born in 1945 have unusually large vocabularies, people born in 1980 have unusually good working memory, and people born in 1990 have unusually fast processing speed. Such concerns can be mitigated by converging results from cross-sectional datasets collected at different times (Schaie, 2005). Here, we compared results derived from Web cross-sectional data with results derived from WAIS-III and WMS-III cross-sectional data collected 20 years earlier. Thus, if the results in Exp. 2 and its replications were driven by cohort effects, all the peaks in these earlier cross-sectional studies should have occurred 20 years younger. Instead, digit span and digit-symbol coding showed similar ages-of-peak-performance in all datasets.

One difference was observed between Web-based and traditional samples: earlier age-of-peak-performance for vocabulary in the latter. This is unlikely to be related to testing method, since it also appears in a long-term paper-and-pencil study (Exp. 4). This novel finding may also explain a current puzzle in the literature: while average vocabulary of both adults and children has increased in recent years, the increase has been much larger for adults than for children, a fact only partly explained by the increase in tertiary education (Flynn, 2010). Our data offer an explanation: vocabulary learning is continuing later into adulthood, possibly due to environmental factors (e.g., continued exposure to new words). Some purchase may be gained by exploring whether other tasks show similar generational changes.

Our findings have practical and theoretical implications. On the practical side, not only is there no age at which humans are performing at peak at all cognitive tasks, there may not be an age at which humans are at peak on most cognitive tasks.