Studies by age group, directionality and quality grading
| Age group | Direction of findings | Number of studies (%) | Quality grading (%) |
|---|---|---|---|
| Infancy ≤1 year of age n=18 | No association between IQ and BF | 9 (50) | A—3 (33.3) |
| B—3 (33.3) | |||
| C—3 (33.3) | |||
| Positive association between IQ and BF | 5 (28) | A—0 (0) | |
| B—1 (20) | |||
| C—4 (80) | |||
| Initial positive association which became negative after adjustment for confounders. | 2 (11) | A—0 (0) | |
| B—1 (50) | |||
| C—1 (50) | |||
| Initial positive association weakened after adjustment for confounders but remained statistically significant | 2 (11) | A—1 (50) | |
| B—0 (0) | |||
| C—1 (50) | |||
| Childhood 1–18 years of age n=70 | No association between IQ and BF | 14 (20) | A—3 (21) |
| B—5 (36) | |||
| C—6 (43) | |||
| Positive association between IQ and BF | 26 (37) | A—10 (38.5) | |
| B—6 (23) | |||
| C—10 (38.5) | |||
| Initial positive association which became negative after adjustment for confounders. | 16 (23) | A—6 (37.5) | |
| B—6 (37.5) | |||
| C—4 (25) | |||
| Initial positive association weakened after adjustment for confounders but remained statistically significant | 14 (20) | A—12 (86) | |
| B—2 (14) | |||
| C—0 (0) | |||
| Adulthood ≥18 years of age n=5 | No association between IQ and BF | 2 (40) | A—0 (0) |
| B—1 (50) | |||
| C—1 (50) | |||
| Positive association between IQ and BF | 1 (20) | A—0 (0) | |
| B—1 (100) | |||
| C—0 (0) | |||
| Initial positive association which became negative after adjustment for confounders. | 1 (20) | A—0 (0) | |
| B—1 (100) | |||
| C—0 (0) | |||
| Initial positive association weakened after adjustment for confounders but remained statistically significant | 1 (20) | A—1 (100) | |
| B—0 (0) | |||
| C—0 (0) |
Infancy: Refs. 22 ,24 ,35–37* 38 ,51 ,54 ,60 ,64* 69 ,70* 72* 73 ,74* 78* 86* 90*
Childhood: Refs. 9 ,13 ,15 ,21 ,23 ,25–34 ,37* 40–43* 45–50 ,52 ,53 ,55–59 61–64* 65–68 ,70* 72* 74* 75–78* 79–82 84–86* 87–90* 91–101
Adulthood: Refs. 39 ,43* 44 ,71 ,83
*Study examined two different age groups and therefore included in several categories.
BF, breastfeeding.