Study sample. Women who were pregnant, residing in the Avon region of the United Kingdom, and expected to deliver between 1 April 1991 and 31 December 1992 were eligible for the Avon Longitudinal Study of Parents and Children (ALSPAC); 14,062 pregnancies were recruited into the study that resulted in live births, and of these, 13,978 were twins or singletons alive at 1 year. The present investigation was restricted to term singletons (n = 12,931) for whom complete infant feeding data were available (n = 8,519) and for whom a play behavior outcome assessment was completed at approximately 42 months of age. The total study sample was 7,076 participants (3,664 boys and 3,412 girls). Mothers provided informed consent for participation. Ethical approval for the study was obtained from the ALSPAC Law and Ethics Committee and the Local Research Ethics Committees. The present analysis was approved by the Institutional Review Board of the University of North Carolina–Chapel Hill.
Exposure assessment. Mothers completed infant feeding questionnaires at 1, 6, 15, and 24 months postpartum. Mothers reported current breast-feeding habits, the age at which other milks or formulas were introduced into the child’s diet (including formula/baby milk, soy milk, soy formula, goat’s milk, follow-on milk, hypoallergenic formula, and cow’s milk), and how many feedings per week were given for each of these products at the time of questionnaire completion.
We defined exposure categories using responses to the questionnaire administered at 6 months postpartum; if these data were missing or incomplete, responses from the 15-month questionnaire were used. “Early” exposure was defined as the use of a specific formula or milk type occurring ≤ 4 months of age [“At what age did you start (formula/milk type)?”] through ≥ 6 months of age. Use at 6 months was indicated by any nonzero response to the question “How often nowadays is your baby fed (formula/milk type)?” in the 6-month questionnaire. If the 15-month questionnaire was used instead, “early” exposure to formula was established for any participant that reported introducing the formula or milk ≤ 4 months of age and responded affirmatively to the question “Since your child was 6 months old, has he/she had (formula/milk type)?” This definition established not only early use of formula but also a 1-month minimum duration of use.
We categorized participants into four mutually exclusive feeding groups: primarily breast-fed, early formula, early soy, and late soy (). Primarily breast-fed infants were those who were breast-fed until ≥ 6 months of age who had no reported introduction of other milks or formulas before 6 months of age and no reported soy milk/formula use before 24 months of age. Early formula-fed infants were those introduced to any nonsoy milk or formula product ≤ 4 months of age, had sustained use of such products at 6 months of age, and no reported soy use before 24 months of age. Early soy-fed infants were those introduced to soy milk or soy formula ≤ 4 months of age who had sustained use at 6 months of age. Late soy-fed infants were those introduced to soy milk or soy formula any time after 4 months of age through 15 months of age. We did not restrict the early formula, early soy, and late soy groups with respect to duration of breast-feeding; likewise, we did not restrict the early or late soy groups with respect to use of nonsoy formula.
Figure 1 Exposure characterization. Participants were classified into four mutually exclusive feeding groups (primarily breast, early formula, early soy, late soy). Participants who did not meet an exposure definition or who had incomplete feeding data were excluded. (more ...)
We excluded participants if feeding profiles were not sufficiently complete to estimate duration of a particular feeding method or if their known exposure profile did not fit into an exposure classification. We also excluded participants who reported soy use only between 15 and 24 months because we assumed that exposure would be low compared with earlier times in infancy. Responses from the 1-month questionnaire were used to verify that no soy was used in early infancy among primarily breast-fed, early formula, and late soy participants. Exposure definitions did not take into account exposure to solid foods or their corresponding soy content, if any.
We used the PSAI, a psychometric test designed to assess within and between gender differences in early-life play (Golombok and Rust 1993a
), to assess gender-role play behavior. To complete the PSAI, mothers or other primary caregivers reported how often their child had played with certain toys (7 items), engaged in certain activities (11 items), and displayed certain characteristics (6 items) for the past month. Half of these items were “masculine,” and half were “feminine.” Each response was scored on a 5-point Likert scale, ranging from “never” to “very often.” The instrument was scored by summing responses to masculine items, subtracting the sum of feminine items, and applying a transformation (48.25 + 1.1 × score) to achieve a “pseudo–t
-score” (Golombok and Rust 1993b
). Higher scores indicate masculine typical behavior, and lower scores indicate feminine typical behavior.
PSAI assessments were administered in ALSPAC at 30, 42, and 57 months of age. We chose the 42-month assessment as our primary outcome of interest, before any analysis, because this age was similar to the age of the sample within which the test was validated (Golombok and Rust 1993b
) and because this time point has been used in previous PSAI publications (Hines et al. 2002a
; Rust et al. 2000
). In post hoc analyses, we used the 30- and 57-month assessments to evaluate consistency of associations over time.
Demographic, family composition, and lifestyle characteristics were assessed through parent report on self-completed questionnaires. The mother’s and partner’s interaction with the child was estimated when the child was 42 months of age using a series of questions assessing the frequency at which each parent participated in a list of eight activities with the child (score range, 0–36). Partner interaction scores were set to zero if the questionnaire reported that no partner was present. We assumed partners were male, given a low prevalence of mothers in same-sex partnerships in this cohort (< 1%) (Golombok et al. 2003
Analysis. Crude mean PSAI scores were assessed as means with 95% confidence intervals (CIs) within exposure groups and within strata of each covariate. Adjusted mean differences in PSAI scores were estimated using multivariable linear regression. Boys and girls were modeled separately to distinguish within-sex differences. The early formula-feeding group was used as the referent. We identified possible confounders as variables thought to be associated with gender role development and statistically associated with any infant feeding method in univariable investigations of these data (chi-square or t-test, p < 0.05, compared with early formula referent). Final models were adjusted for age at assessment, presence of an older brother (yes/no) or an older sister (yes/no), regular child-care attendance (yes/no), maternal and partner interaction scores, and maternal factors, including age at delivery, smoking in the third trimester (yes/no), and education [ranked from high to low: university degree, advanced level, ordinary level, vocational, and Certificate of Secondary Education (CSE)/none].
Data were analyzed using both complete case analysis and multiple imputation. For the complete case analysis, participants with missing data for adjustment variables (~ 18%) were dropped from models, so only true values for participants with complete data were modeled. All analyses were completed using SAS (versions 9.1.3 and 9.2; SAS Institute Inc., Cary, NC). For multiple imputation models, values for missing covariates were estimated from available data on all adjustment variables, as well as breast-feeding duration and marital status, using PROC MI (five iterations). Regression models of imputed data were run and summarized in PROC MIANALYZE.