The research has some serious limitations. The possible unknown factors are driving differences in amounts of touch and in the children’s brain development like genes.

Bigelow, A. E., Power, M., MacLellan-Peters, J., Alex, M., & McDonald, C. (2012). Effect of mother-infant skin-to-skin contact on postpartum depression and maternal physiological stress.  Journal of Obstetric, Gynecologic, and Neonatal Nursing, 41, 369-382.