Table 1.

Associations Between Duration of Breastfeeding and Measures of Cognitive Ability, Teacher Ratings of School Performance, Standardized Tests of Achievement, and High School Success

MeasureNDuration of BreastfeedingrP
Not Breastfed<4 Months4–7 Months≥8 Months
Mean cognitive ability scores*
 WISC–R total IQ, 8 years86997.4698.89102.16102.830.22<.0001
  9 years79997.7898.55102.39102.800.21<.0001
Mean teacher ratings of achievement
 Reading, 8 years10642.852.943.083.200.14<.0001
  12 years9882.762.923.183.270.20<.0001
 Mathematics, 8 years10642.892.883.043.250.14<.0001
  12 years9812.732.973.123.270.20<.0001
Mean standardized achievement test scores*
 Reading comprehension, 10 years83497.8698.94101.77103.060.21<.0001
  12 years79198.0598.82100.87103.490.20<.0001
 Mathematics, 11 years81898.1698.81101.19102.900.18<.0001
 Scholastic ability, 13 years77297.7899.00101.45103.050.21<.0001
High school attainment
 Mean number of School Certificate passes 9992.663.043.753.990.24<.0001
 Leaving school without qualifications (%)99925.325.510.89.7−0.18<.0001
Percentage of sample in each breast feeding group31.1%26.6%18.4%23.9%
  • * Measures of cognitive ability and standardized tests of achievement have been scaled to have a mean of 100 and an SD unit of 10.

  • Teacher ratings have been scaled to have a mean of 3 and an SD unit of 1.

  • The sample SD for the number of School Certificate passes was 2.27.