There is a study out of the department of psychiatry at Massachusetts General Hospital
that says that parents of youngsters who were breast-fed as infants were less likely to report that their child had a behavior problem or psychiatric illness during the first five years of life. The study goes on to say that the likelihood of mental health issues decreased in proportion to the duration of breast-feeding, meaning that a child who had been breast-fed for a year was less likely to have behavior problems than a child who had been breast-fed for just two months.
This study used more than 100,000 interviews of parents and guardians of children between the ages of 10 months and 18 years who participated in the National Survey of Children's Health. Parents were asked about breast-feeding and about their child's behavior and mental health.
Don't get me wrong. I am in no way against breast feeding.
I did it for both of my kids because in my opinion, it was a no-brainer for my situation. While breast feeding may not be for everyone, there are plenty of studies and opinions and it's benefits.
Having said that, I can also tell you that as much as I love him, my son was not always the best behaved. He was breastfed for a year. He was always very 'active' until about the 5th grade. After all, he's a boy. Young boys are curious, active and told by many in society that they should be somewhat adventurous.
Breast feeding definitely has it's benefits.
However, I'm not sold on the good behavior theory.