See: Bonding (attachment) Breastfeeding & Bottle-feeding Violence
In a series of subsequent cross-cultural tribal studies, Dr. Prescott found that 77% of 26 tribal cultures whose weaning age was 2.5 years or longer were rated low or absent in suicidal violence. Further, he found significant differences in suicidal behaviors between cultures with weaning age of 2.0 years or less v 2.5 years or greater. This finding suggests that a critical period of brain development exists at this age to mediate this effect. These and other data suggest that breastfeeding for 2.5 years or longer is required to optimize the health benefits of breastfeeding for child and mother (Zheng, 2000). These breastfeeding effects are undoubtedly mediated, in large part, by the rich presence of the amino acid tryptophan in breastmilk that is deficient in infant formula milk and which is necessary for normal brain serotonin development  The Origins of Love & Violence: An Overview by James W. Prescott, Ph.D.
Barry and Paxon (1971) provided the ranges of weaning age for 186 cultures, among a number of other behavioural codes on infancy and childhood, which I utilised to extend my studies on weaning age and lack of suicide. The average weaning age of each culture was calculated from the range scores given that permitted the statistical studies on those cultures whose weaning age was 24 months or less v 30 months or more. These data were added to those reported by Textor (1967), which yielded 65 cultures where information was available on both weaning age and suicide. It was found that 64% (23/36) of cultures were rated low or absent in suicide with a weaning age of 30 months or greater; 62% (18/29) of cultures with high suicide rates had a weaning age of 24 months or less. This difference was statistically significant (p < .05).  James W Prescott, Ph.D. Why breastfeeding mothers are important
Mothers, by breastfeeding their children (male and female) for 2.5 years and beyond, can radically alter human societies, reduce depression and violence by over fifty percent, and pave the way for true human equality.  James W Prescott, Ph.D. Why breastfeeding mothers are important