Although a substantial relationship of sustained attention and reasoning was consistently found, little is known about what drives this relationship. The present study aims at revealing the underlying sources that are responsible for the relationship by means of an integrative approach combining experimental manipulation and psychometric modeling. A sample of 4551 participants falling into three age groups (aged 12, 13, and 14 years) completed a sustained attention test and three reasoning subtests. The attention test comprised four treatment levels designed by manipulating the number of distracters. Confirmatory factor models were applied to decompose the variance of the attention data into three components: executive control due to stimulus manipulation, automatic learning as the result of extended practice, and a general component independent of any changes. The results showed that in all age groups the component of execution control showed the strongest link with reasoning although the learning component also significantly related with reasoning. In contrast, a non-significant link was observed between the general component and reasoning. These results indicated that the sources of the relationship between sustained attention and reasoning are mainly due to executive control of attention, and additionally to a learning process. (Contains 4 tables and 2 figures.)