We used here a latent growth curve modeling approach to test the association of breastfeeding with IQ growth trajectories, which allows differentiating the variance in the IQ starting point in early life from variance in IQ gains that occur later in childhood through adolescence. Breastfeeding (yes/ no) was modeled as a direct predictor of three IQ latent growth factors (i.e. intercept, slope and quadratic term) and adjusted for the covariates socioeconomic status, mother's age at birth and gestational stage. Data came from the Twins Early Development Study (TEDS), a prospective cohort study of twins born between 1996 and 1994 in the United Kingdom, who were assessed 9 times on IQ between age 2 and 16 years (N = 11,582).

Introduction

Many empirical studies have investigated if being breastfed in early life benefits later cognitive development [1]. An association between breastfeeding and cognition is plausible because long-chain polysaturated fatty acids (PUFA), which are present in human breast milk but not in milk from animals or formula, enhance neurodevelopment [2]. However, the evidence on the association between breastfeeding and cognitive growth is to date inconsistent and inconclusive, with some studies reporting a positive relationship and others failing to detect such effects after adjusting for relevant covariates. A recent comprehensive review of 84 relevant studies on breastfeeding practices and intelligence concluded that any observed associations between the two were best explained by residual confounding [1]. Thus, the relationship between breastfeeding and intelligence may not be causal in nature but instead reflected the interconnection of favorable variables associated with breastfeeding, such as advantaged family socioeconomic status (SES) and higher parental intelligence [3].

To disentangle ‘true’ and ‘confounding’ effects in the breastfeeding-intelligence association, it is important to differentiate children’s differences in early life intelligence–that is, at the beginning of their cognitive growth trajectories–from the differences that children show in cognitive growth or intelligence gains over time [4]. If there were nutritional benefits of breastfeeding for cognitive growth, breastfeeding should be more strongly associated with early life cognitive ability or IQ starting points but to a lesser degree with children’s later intelligence gains [4]. However, if breastfeeding was mainly related to long-term cognitive development rather than initial intelligence, the association was likely to result from confounding and be attributable to breastfeeding’s interrelatedness with other variables that exert favorable influences on development [4]. Huang and colleagues tested this hypothesis recently in data from the Child Development Supplement of the Panel Study of Income Dynamics (CDS), a panel sample of 2,784 children, who were initially aged 0 to 12 years and assessed up to three times on cognitive ability over a study period of 10 years. Breastfeeding was modeled as a dichotomous predictor of children’s cognitive ability test scores at each assessment wave, adjusting for relevant covariates. The results showed that children who had been breastfed had significantly higher initial intelligence test scores than those who had not been breastfed. However beyond that, breastfeeding was not associated with cognitive growth trajectories. That is, the long-term cognitive development of breastfed and not breastfed children was similar, supporting the nutritional benefits hypothesis, even though the initial breastfeeding gap persisted over time. Huang and colleagues concluded that breastfeeding had a ‘true’ effect on cognitive development that was not eliminated by later life experiences and thus, was not to be attributed to residual confounding.

In the current study, we seek to replicate and extend Huang and colleagues’ earlier findings, overcoming four limitations of their original investigation. First, the CDS study sample included children with a wide age range (0 to 12 years); thus, early IQ development could only be studied for part of the sample. By comparison, we analyze data from the Twins Early Development Study (TEDS), a prospective cohort twin sample that is demographically representative of the United Kingdom and whose members were born between 1994 and 1996 [5]. Second, the intelligence of the CDS children was assessed at most three times and at different ages during the 10-year study period of 10 years, but for some children only one or two valid cognitive scores were available [4]. By comparison, TEDS children were tested 9 times on intelligence between the ages of 2 and 16 year; thus, they were assessed at the same ages and at least three times during the 14-year study duration. Also, TEDS includes a much larger sample of children than CDS (11,582 vs. 2,784). Our large, longitudinally assessed sample allows testing latent growth curve (LGC) models, which differentiate variance in latent growth factors that represent children’s differences in early life intelligence and systematic changes or gains in intelligence over time [6]. Third, data on breastfeeding practices in CDS were collected from mothers at the study’s first assessment wave, that is, 0 to 12 years after the actual event. Mothers’ recall of breastfeeding has been shown to be most reliable and valid within the first three years the event [7]. In TEDS, breastfeeding data also rely on mothers’ recall at the first assessment wave, but this took place within two years of the twins’ birth, suggesting that the breastfeeding data used in the current study will be less affected by measurement error. Fourth, twins’ cognitive development is similar to that of singletons [8], and twin studies make it possible to confirm results by testing two directly comparable samples of siblings, each consisting of one twin randomly chosen from a pair. This possibility of direct replication is not provided by cohort studies of unrelated children. Finally, we also explore here gender differences in our LGC analysis of cognitive development in TEDS, which had not been previously addressed.