Variation in breast milk quality affects infant behavior

In a study of rhesus maqaques, moms who were larger and who had had previous pregnancies gave more milk and better milk, with more "milk energy." At 3 to 4 months old, the separated infants from mom.

The study found that infants whose mothers had higher levels of milk energy soon after their birth coped more effectively (moved around more, explored more, ate and drank) and showed greater confidence (were more playful, curious and active). Infants whose mothers had lower milk energy had lower activity levels and were less confident when separated from their mother.

Okay, here's a dissertation waiting to be written, analyzing human breast milk and infant behavior.