The socioeconomic status of a child could influence both their type of family attachment as well as their well-being later in life. Attachment and well-being would therefore be correlated, but not causally related.
I think that while this may be true sometimes, it would in no mean be a strong correlation. Without a doubt, statistically, socioeconomic status may influence a child's well-being later in life. However, poor socioeconomic status I feel would not be as strongly correlated with a babies' attachment to parent; perhaps in some situations, but certainly not enough to make a conclusion.
Quite the opposite, I feel that a poor socio-economic status may be a factor that leads to attachment and later well-being behaving opposite as expected. Which is to say, a baby has a secure attachment to his or her parent, but due to conditions growing up does not do well later in life.
