Socioeconomic status, parental education, and parental intelligence have strong effects on child IQ and are themselves correlated with breastfeeding practices. When studies ignore these confounders, they often find significant IQ increases in breastfed groups (5-8 points).

As more confounders are accounted for, the gap typically shrinks to 2-3 points. Some relevant studies:

• Der, Batty & Deary, 2006: 5475 children, controlled for most relevant factors including maternal IQ → + ~0.5 IQ points
• Strøm et al., 2019: 1782 children, controlled for most relevant factors including maternal IQ → + ~3 IQ points
• Pereyra-Elías, Quigley & Carson, 2022: 7855 children, controlled for most relevant factors including maternal IQ → + ~4 IQ points

However, the gold standard are sibling studies, where breastfed and non-breastfed siblings, who share the same parents and home environment, are compared. Here the effect shrinks further, though doesn’t go to zero.

• Evenhouse & Reilly, 2005: 2734 sibling pairs → +~0.5 IQ points
• Colen & Ramey, 2014: 7319 children; 1773 sibling groups → No effect measured
• Sanefuji et al., 2021: 3521 duos or trios of siblings → Longer/continuous breastfeeding linked to fewer developmental delays, even when sibling-controlled, but IQ not measured
• Goldshtein et al., 2025: 570,532 children, 37,704 sibling pairs → Longer/continuous breastfeeding linked to fewer developmental delays, even when sibling-controlled, but IQ not measured

The largest sibling-controlled study (Goldshtein et al.) unfortunately did not measure IQ. However, they did find that breastfeeding for at least 6 months is linked to about ~1-2 fewer children with developmental delays per 100, and ~7-13 fewer per 1000 with neurodevelopmental conditions. This was also confirmed in their sibling comparisons.