This paper examines evidence for relationships among individual differences (IDs) in L1 achievement, L2 aptitude, and L2 achievement. It begins by providing an overview of my academic journey from L1 educator to L2 researcher and explains the development of the Linguistic Coding Differences Hypothesis (LCDH). Next, I review Peter Skehan's influential research on language aptitude and L1-L2 relationships and summarizes research showing that IDs in L1 achievement (and L1 ability) are universal, can be identified early, and are stable over time. Then, I review research on L1-L2 relationships and IDs in L2 learning, focusing on the longitudinal studies and reporting two new studies that examine the relationship between L1 achievement and L2 aptitude on the MLAT. Overall, I argue that the development of L1 literacy leads to growth in metalinguistic awareness, which enhances L1 literacy skills, and that L1 literacy and metalinguistic awareness provide the foundation for L2 aptitude, the ability to use and understand "decontextualized" material. I conclude the paper by proposing an answer to the title.