Does Breastfeeding Make Kids Smarter?

“Breast is best” includes, in most parenting guidance, a confident claim that breastfeeding makes children smarter. The observational data are real — children who were breastfed score about 3 to 4 IQ points higher on cognitive tests than children who were not. The harder question is whether the milk does this or whether the kinds of mothers who breastfeed differ from the kinds who don’t in ways that themselves predict child IQ. Sixteen years of follow-up on the only large-scale randomized trial in this area, plus sibling-comparison studies that strip out shared family confounders, now suggest that the causal effect of breastfeeding on full-term children’s intelligence is small — likely 1 to 2 IQ points, and possibly indistinguishable from zero on most measures by adolescence. The cognitive advantage is real and bounded, not the headline benefit of breastfeeding.

What the observational studies show

Dozens of observational studies report a breastfeeding-IQ advantage, and the meta-analytic estimate is consistent across decades. Horta, Loret de Mola, and Victora’s 2015 systematic review and meta-analysis in Acta Paediatrica reported an unadjusted difference of about 3 to 4 IQ points — the same range Der, Batty, and Deary had reported in 2006 in BMJ with a meta-analysis of 17 studies (pooled +3.44 IQ points, 95% CI 2.30–4.58). When studies controlled for maternal IQ, the same meta-analytic pooling dropped to +2.62 points (1.25–3.98). The drop is the first hint that confounding is doing meaningful work.

Why this question is hard: the maternal-IQ confound

The deepest problem in the breastfeeding-IQ literature is selection. Mothers who breastfeed differ systematically from mothers who don’t, and several of those differences predict child IQ independently of feeding method.

The single largest confounder is maternal IQ. Der, Batty, and Deary (2006) found that one standard deviation higher maternal IQ more than doubled the odds of breastfeeding — a stronger predictor than maternal race, education, age, poverty status, smoking, the home environment, or the child’s birth weight. Since adult IQ is roughly 50–80% heritable, the children of higher-IQ mothers will score higher on cognitive tests regardless of feeding method.

Other confounders compound the problem:

• Socioeconomic status: Breastfeeding rates are higher among college-educated, higher-income families. These families also provide more cognitively stimulating home environments.
• Parenting quality: Breastfeeding is associated with more responsive parenting, more physical contact, and more face-to-face interaction — all of which independently support cognitive development.
• Health behaviors: Breastfeeding mothers are less likely to smoke or drink during pregnancy; healthy pregnancy nutrition tracks similarly.

When researchers control for maternal IQ alone, the breastfeeding-IQ association typically shrinks by half or more. When they additionally control for SES and home environment, the residual is often statistically indistinguishable from zero.

The strongest causal evidence: PROBIT, then PROBIT-at-16

The Promotion of Breastfeeding Intervention Trial (PROBIT) is the only large-scale randomized trial of breastfeeding promotion. Conducted in Belarus, it cluster-randomized 31 maternity hospitals (with 17,046 mother-infant pairs) to either a Baby-Friendly Hospital Initiative breastfeeding-support intervention or standard care. The intervention raised exclusive breastfeeding at 3 months from 6.4% in the control group to 43.3% in the intervention group — a real causal increase in breastfeeding exposure.

Kramer and colleagues’ 2008 paper in Archives of General Psychiatry reported the cognitive outcomes at age 6.5: cluster-adjusted mean differences favoring the intervention group of +7.5 verbal IQ, +2.9 performance IQ, and +5.9 full-scale IQ. This is the strongest single piece of causal evidence on breastfeeding and child cognition. It pushed the literature toward a confident “breastfeeding raises IQ” reading for about a decade.

That reading has not held up. Yang and colleagues’ 2018 paper in PLOS Medicine, the 16-year follow-up of the same PROBIT cohort (with 79.5% retention — 13,557 of the original 17,046 participants), found that the breastfeeding-promotion effect on overall neurocognitive function had become statistically non-significant by adolescence. The only domain showing a persistent benefit was verbal function, with a difference of 1.4 points at age 16. The performance and full-scale advantages of the original 6.5-year analysis had largely washed out by adolescence.

The trajectory matters more than either snapshot. PROBIT remains the only randomized evidence in this area. Its longest follow-up tells us that whatever causal effect breastfeeding has on cognition is real but small in early childhood, and it shrinks substantially as children move into adolescence and the broader environment carries more of the cognitive load.

Sibling-comparison studies: confounding does most of the work

Sibling-comparison designs control for shared family characteristics — maternal IQ, SES, parenting style, home environment — that are otherwise nearly impossible to fully measure. They compare breastfed children to their formula-fed siblings within the same family.

The largest sibling-comparison study in this literature is Colen and Ramey’s 2014 paper in Social Science & Medicine, drawing on more than 8,000 children from the National Longitudinal Survey of Youth (NLSY). After sibling-comparison analysis, breastfed and non-breastfed siblings did not differ significantly on cognitive outcomes. Der and colleagues’ 2006 sibling-pair analysis reached the same conclusion. Two large independent sibling designs converging on the same answer is a strong signal that observational breastfeeding-IQ effects in the broader literature are largely confounded.

The preterm exception

The full-term-versus-preterm distinction matters. Maternal milk feeding in preterm infants shows substantially stronger cognitive and academic effects, with a clearer dose-response relationship. The likely reasons: preterm infants face concrete medical risks (necrotising enterocolitis, sepsis) that maternal milk reduces; their brains are still actively developing during the NICU period in ways that full-term brains are not; and the mechanistic pathway through infection-and-NEC reduction is well-documented in this population. Preterm infants are the population where the maternal-milk-and-cognition story is most empirically secure. Generalizing from there to full-term, well-nourished infants overstates what the evidence supports.

How big is the real effect for healthy full-term infants?

Synthesizing across the strongest study designs:

• Observational meta-analyses: +3 to +4 IQ points unadjusted; +2 to +3 with maternal IQ control.
• PROBIT at age 6.5 (Kramer 2008, randomized): +5.9 full-scale IQ — strongest single causal estimate.
• PROBIT at age 16 (Yang 2018, same cohort): +1.4 verbal function only; non-significant on overall neurocognitive function.
• Sibling comparisons (Der 2006; Colen & Ramey 2014): No significant difference within families.

The honest landing point: for full-term, well-nourished infants, the long-term causal effect of breastfeeding on cognitive function is likely 0 to 2 IQ points — small enough to be clinically meaningless at the individual level, and possibly zero on most measures by adolescence. The persistent advantage that the strongest randomized evidence supports is concentrated in verbal function and is on the order of 1 to 2 points, not the 5+ points that the early-childhood analyses suggested.

Why this matters

The breastfeeding-IQ question matters not because two points of IQ is consequential for any individual child, but because of how it shapes the conversation we have with parents. Overstating the cognitive benefits of breastfeeding produces guilt and anxiety in mothers who cannot or choose not to breastfeed, and it diverts attention from interventions with stronger evidence — quality home learning environments, shared reading, prenatal nutrition, and reducing exposure to known cognitive harms.

Breastfeeding has well-documented benefits on infection rates, maternal cancer risk, and other health outcomes that justify recommending it. Adding “raises IQ” to that list is no longer defensible at the strength the popular framing assumes. The honest message to parents is that breastfeeding has real benefits — but for full-term, healthy infants, raising the child’s intelligence by any meaningful amount is probably not one of them.

Frequently Asked Questions

Does breastfeeding really raise a baby’s IQ?

Observational studies consistently find a 3–4 IQ-point advantage in breastfed children, but the strongest causal designs (PROBIT 16-year follow-up, sibling comparisons) suggest the long-term causal effect for full-term infants is small — roughly 0 to 2 points, concentrated in verbal function. The early-childhood difference of about 6 IQ points seen in PROBIT at age 6.5 had shrunk to 1.4 verbal-function points by age 16.

What’s the maternal IQ confound?

Mothers with higher cognitive ability are substantially more likely to breastfeed. Since adult IQ is 50–80% heritable, their children will score higher on cognitive tests regardless of feeding method. Der, Batty, and Deary (2006) found maternal IQ was a stronger predictor of breastfeeding than race, education, or socioeconomic status — controlling for it cuts the observational advantage roughly in half.

Is the evidence different for preterm babies?

Yes. Preterm and low-birth-weight infants show substantially stronger cognitive benefits from maternal milk, with mechanisms running through reduced infection and necrotising enterocolitis as well as direct nutritional support during a developmentally critical period. The full-term-vs-preterm distinction matters; conclusions from one population don’t translate cleanly to the other.

If I can’t breastfeed, should I worry about my child’s intelligence?

The strongest current evidence does not support meaningful long-term IQ differences between breastfed and formula-fed full-term children once confounding is properly controlled. The home environment, shared reading, conversation, and adequate prenatal nutrition are larger and more modifiable contributors to cognitive development than feeding method.

What does the evidence support promoting breastfeeding for?

Lower rates of infant infection, reduced risk of necrotising enterocolitis in preterm infants, lower maternal breast and ovarian cancer risk, and metabolic and immune benefits with substantial evidence. These justify continued promotion of breastfeeding when feasible. The cognitive case is weaker than these other benefits and shouldn’t be the headline reason given to parents.