We examined the genetic architecture of a Tower of London test of planning and problem-solving in 690 middle-aged male twins. Phenotypic analyses revealed only one general factor, but the best-fitting genetic model indicated two correlated genetic factors: speed and efficiency. One variable-number of attempts required to mentally figure the puzzles-loaded on both factors. Shared environmental effects could be dropped with virtually no reduction in model fit. Despite significant nonshared environmental correlations across measures, there was no discernable nonshared environmental factor structure. The correlation between genetic factors (r = 0.46) and the variable loading on both factors could reflect modulation of planning, testing alternatives, and working memory that are required to perform the test. Such coordinated activity is consistent with the notion of a supervisory attentional system, a central executive, or metacognitive ability. The different phenotypic and genetic factor results suggest that relying solely on the former could obscure genetic associations.