By the time children start formal education there are already substantial individual differences in their mathematical skills (e.g., Ginsburg, Lee, & Boyd, 2008;Klibanoff et al., 2006). Because these individual differences are predictive of later school achievement (e.g., Jordan, Kaplan, Ramineni, & Locuniak, 2009), they merit serious research attention. In particular, what factors influence the level of mathematical skill a child arrives at school with? Of course, there are many plausible hypotheses: different levels of parental engagement, differences in children's domain general skills, differences in socioeconomic factors, and so on. In 2005 Hannula and Lehtinen put forward the exciting hypothesis that another possible source of individual differences in children's mathematical achievement is the extent to which they focus, unprovoked, on numerical aspects of their environment. suggested that children with a high Spontaneous Focusing on Numerosity (SFON) tendency are more likely to, across a whole range of situations, notice the numerical aspects in their environments. This tendency gives them more practice at performing mathematical tasks, such as converting non-symbolic numerosity representations into symbolic numbers. For example, a child with a high SFON tendency might, while playing in the park, notice that there are four dogs and think to themselves "aha, there are four dogs!" In contrast, a child with a low SFON tendency might think "aha, those dogs are all brown!" Over time, this kind of self-initiated numerical