“Breastfed babies less likely to grow up angry and irritable,” according to the Daily Mail. The newspaper said that a long-term study of a group of breastfed babies has found they grew up to be less hostile than a group of bottle-fed counterparts.
The news is based on a long-running Finnish study that had followed nearly 2,000 people from childhood until their 30s. The researchers found that, compared to participants who had never been breastfed, those who had been breastfed for four to six months had lower scores on hostility tests as adults. While the researchers found a difference of around 0.2 points, they do not explain whether there is any clinical relevance in this difference in scores, and it may be the case that this difference would not be noticeable in real life.
This study has a number of limitations, one of which is that it does not look at the reasons why mothers chose to breastfeed or not. Therefore, it is not possible to fully explore the reasons for the link. Mothers are encouraged to breastfeed where possible for the numerous known health benefits for their baby. However, this study requires further follow-up before it is possible to say whether it can have long-term psychological benefits too.
Where did the story come from?
The study was carried out by researchers from the Finnish Institute of Occupational Health in Helsinki and Turku University, also in Finland. It was funded by numerous Finnish funding bodies and published as a letter to the editor in the peer-reviewed journal Psychotherapy and Psychosomatics.
The Daily Mail and the Daily Express did not mention that the researchers estimated that breastfeeding only had a small effect, or the limited relevance the effect was likely to have. The Daily Mail also suggested that hostility was measured when the participants were 24 years of age. In fact, the participants were followed for a period at least 24 years and had their hostility measured on numerous occasions, including when they were in their 30s.
What kind of research was this?
This prospective cohort study followed a group of children and adolescents into their thirties. Its aim was to see whether there was an association between breastfeeding and psychological development and behaviour, particularly hostility.
The researchers said that cold and unsupportive parenting has been associated with children developing hostility, but that no study had specifically looked at the effect of breastfeeding. This study did not look at the reason why women breastfed or did not.
What did the research involve?
The researchers randomly selected 1,917 Finnish children and adolescents considered to be a nationally representative sample of the population. The participants had been born at full term (none were premature) and weighed over 2.5kg at birth.
In 1983, when the children were on average 12.6 years of age, their parents were asked about their child’s breastfeeding history. In Finland, women keep records of their breastfeeding in record cards and the researchers also checked these to verify their data.
The researchers assessed measures of hostility in 1992, 1997, 2001 and 2007 when the participants were on average 21.5, 26.7, 30.8 and 36.9 years of age. Hostility was measured using three scales assessing:
A total hostility score was calculated by taking the average score on these three scales.
What were the basic results?
The researchers found that 88% of the participants had been breastfed and were, on average, breastfed for four months.
They found that, overall, the mothers were on average 27 years old when they gave birth, but the average age of the group of mothers who did not breastfeed was 29.6 years when they gave birth. They also found that older mothers that breastfed tended to do so for longer, and that longer duration of breastfeeding was related to:
- less hostile maternal child rearing practices (not further specified in the study)
- lower family income
- higher number of children in the family
- a later ‘birth order’ of the child, that is, to be younger within a group of siblings, which some argue can be psychologically influential
The researchers found that out of these family characteristics, hostility of the offspring was associated with:
- hostile child rearing
- low family income
The researchers then looked at the hostility scores in adulthood of participants who had been breastfed for four to six months when they were babies and compared them to those of subjects who had not been breastfed. They adjusted the data for age, gender, mother’s age when she had her child, maternal education, family structure, family income, number of children in their family, birth order and their birth weight.
They found that on average the participants who were not breastfed had a total hostility score of 2.67 (95% confidence interval [CI] 2.57 to 2.78). The average score among the participants who had been breastfed for four to six months was lower, at 2.49 points (95% CI 2.43 to 2.55).
The researchers had compared three different scales to get the total score but did not report the range of the scales. It is not clear whether an approximately 0.2 point difference between the breastfed and non-breastfed groups is particularly large or has any real-life meaning - in other words, whether this difference means that the participants’ hostility had any impact on their life or of those around them.
How did the researchers interpret the results?
The researchers said their study showed that breastfeeding may have long-term effects on offspring hostility and that ‘those who had not been breastfed as infants had higher levels of hostility, especially cynicism and paranoia, in adulthood than their four to six month breastfed peers’.
This Finnish prospective cohort study investigated whether breastfeeding compared to non-breastfeeding was associated with lower hostility in adulthood.
While the research featured some laudable methods, such as assessing participants multiple times over a long study period, its results are somewhat unclear. The difference between the average hostility scores was reported as being just under 0.2 points, but the clinical significance (if any) of this difference was not described. As such, it is not clear whether this difference would have any noticeable effect on the life of the person or the people around them.
The researchers themselves acknowledged other limitations of this study:
- as breastfeeding was self-reported parents may have remembered inaccurately or said that they breastfed when they did not, perhaps if they thought that this was a more socially desirable answer
- the most disadvantaged participants dropped out of the study
Crucially, this study did not ask mothers who had not breastfed why they did not do so. Without this the study cannot fully explore potential reasons for a theoretical link. We cannot tell if breastfeeding might produce some biological change that affects hostility or whether breastfeeding is associated with social factors that might also shape personality.
Mothers are encouraged when possible to breastfeed for the known health benefits for their baby, and for the close physical and emotional mother-baby relationship that breastfeeding supports. However, this study needs further follow-up before it is possible to say whether there are additional long-term psychological benefits too.