Assuming that attachment is the IV, and success in later life is the DV, then you can find a correlation between the two. However, this does not mean that there is necessarily a causal effect. Another factor could come into play. For instance, what if there is another variable that causes both secure attachment and success in later life? For instance, a parent who plays lots of games with an infant. This play time could cause both attachment and success later in life from improved social and problem solving skills. Alternatively, a lack of an affectionate and playful parent could cause attachment issues, and problems later in life.
