In recent years, researchers have begun to identify cross-cultural and within-cultural variation with respect to several domains of human cognition. These include the effects of language background on executive control, differences in the attentional patterns of individuals in the United States and East Asia, and the relationship between personality and the use of language. The underlying mechanisms for how cognitive variation arises is likely due to interactions within a complex system that spans the cognition of individuals and the dissemination of information at a cultural level. This culture-cognition system can be seen as dynamic system of cognitive tools, instruments – biological or technological – through which people interact with and understand the world. Each individual acquires a unique bias for using particular cognitive tools – or a cognitive style. Differences in cognitive style arise for two reasons. First, individuals who are exposed to different cognitive tools will acquire different cognitive styles. Second, individuals may need to strengthen certain preexisting cognitive tools if they cannot optimally support the acquisition of a new cognitive tool. In this dissertation, I argue that the understanding of cognitive tools and styles is critical for research on cognitive variation. I illustrate this point with three case studies that examine the effects of language learning on executive control, US-Japan differences in attentional style, and the relation between personality and vocabulary size.