The number of weeks a child is breastfed may be associated with intelligence in later life, according to a new study, which indicates that IQ rises as the number of breastfed weeks increases, until the effect levels off after nine months.
The study involved more than 3,250 men and women and appears in the Journal of the American Medical Association.
Two IQ tests were conducted, using two separate groups, and average IQ increased in both as the period of breastfeeding rose.
All of the participants were among the 9,125 people in the Copenhagen Perinatal Cohort, a group of individuals in Denmark who feature in a range of medical and social studies. All were born between 1959 and 1961.
One group of 2,280 men were given the Borge Priens Prove intelligence test and another group of 973 men and women took the Wechsler Adult Intelligence Scale test.
Participants in the study were broken into groups on the basis of how long they were breastfed - measured as less than one month, two to three months, four to six months, seven to nine months and more than nine months.
The researchers took account of other factors which might have had an influence on later IQ.
These included parental social status and education, single-mother status, the age of the mother, smoking habits, weight gain during pregnancy, number of pregnancies and any complications during delivery.
"Independent of a wide range of possible confounding factors, a significant positive association between duration of breastfeeding and intelligence was observed in two independent samples of young adults assessed with two different intelligence tests," the researchers from the Copenhagen University Hospital and the Kinsey Institute for Research in Sex, Gender and Reproduction at Indiana University found.
IQ ratings in the mixed-sex group averaged 99.4 for those breastfed for one month or less. It rose to 101.7 for those breastfed for two to three months, to 102.3 for four to six months, peaking at 106 for seven to nine months.
It fell back to 104 for those breastfed for more than nine months, according to the research team.
The researchers come up with three possible reasons for the findings.
Breast milk may contain nutrients not found in cow's milk or formula which stimulate brain development.
The physical and psychological contact between mother and child may be the influencing factor.
They also suggest that there may be some other, unidentified factors which connect infant feeding methods and later IQ.