Research has shown that educated children and adults have access to two ways of representing numerical information: an approximate number system (ANS) that is present from birth and allows for quick approximations of numbers of objects encountered in one's environment, and an exact number system (ENS) that is acquired through experience and instruction, that requires an understanding of language and symbols, and that is at the core of school math abilities. While these two systems are distinct, individual differences in the acuity of the ANS predict later math abilities and advancing the ENS leads to increases in the acuity of the ANS suggesting a reciprocal connection between the two systems. Recently, the focus of the field has turned toward elucidating the mechanisms that underlie this connection, but more work is also needed to understand the sources of individual differences in the ANS and ENS in the first place.