Difficult Temperament, Breastfeeding, and Their Mutual Prospective Effects: The Norwegian Mother and Child Cohort Study Susan Niegel, MSc,* Eivind Ystrom, MSc,* Knut A. Hagtvet, PhD,† Margarete E. Vollrath, PhD*† ABSTRACT: Objective: (1) To examine the extent to which difficult temperament and breastfeeding are associ- ated at child age 6 and 18 months. (2) To examine longitudinally whether breastfeeding has an influence on temperament development or whether difficult temperament has an influence on continued breastfeeding. Method: This prospective study is part of the Norwegian Mother and Child Cohort Study, which targets all women giving birth in Norway; the present sample comprises 30,466 children. Mothers reported on child difficult temperament at child age 6 and 18 months and on breastfeeding from 0 to 6 months and from 6 to 12–14 months. Prospective associations between breastfeeding and difficult temperament were examined using structural equation modeling, with comparison of cross-lagged pathways. All analyses were adjusted for background variables that are relevant for breastfeeding. Results: At 6 months, children with more difficult temperament were significantly less likely to have been “fully” breastfed (exclusively and predominantly breastfed) for the recommended period of 6 months (unadjusted point-biserial correlation r pb .15; adjusted r pb .11). At 12 to 14 months, there was no longer an association between difficult temperament and (continued) breastfeeding after adjusting for background variables, temperament, and breastfeeding at 6 months. The cross-lagged analyses of the longitudinal pathways yielded negligible effects of difficult temper- ament on later breastfeeding and of breastfeeding on later temperament. Conclusion: Difficult temperament and reduced breastfeeding seem to be associated only during the first half-year of the child’s life. After that, we found no evidence of codevelopment between difficult temperament and breastfeeding. (J Dev Behav Pediatr 29:458 –462, 2008) Index terms: difficult temperament, breastfeeding, prospective cohort study. Today, the World Health Organization recommends 6 months of exclusive breastfeeding as the optimal form of infant nutrition and (in developed countries) continued breastfeeding, along with feeding solid foods, throughout the entire first year of life and possibly longer. 1,2 Breast- feeding is supposed to foster infant health and to have positive effects on the child’s psychological develop- ment—though it is unclear whether this would be medi- ated through nutritional or psychological mechanisms. Several researchers reported that breastfed babies are temperamentally “easier,” less irritable, more positive, and more active and sociable later in infancy, claiming that breastfeeding has a beneficial effect on temperament development. 3–6 However, the results are inconsistent, and since most of these studies were conducted in small, selected samples and had a cross-sectional de- sign, conclusions with regard to causal effects of breastfeeding on infant temperament are not war- ranted. It is equally possible that child temperamental traits present since the newborn stage alleviate or im- pede breastfeeding. 3,4,7–9 Subjectively, mothers find it difficult to breastfeed babies that fuss and cry a lot and may feel tempted to give additional foods earlier to soothe the child. 9,10 To advance knowledge about the supposed association between breastfeeding and difficult temperament, larger studies with a prospective design are necessary. The present study is especially suited for this purpose, as it comprises a large population sample of mothers and ba- bies that were followed prospectively from birth to child age 18 months. Two research questions will be addressed in the following: 1. To what extent are difficult temperament and breast- feeding associated when the child is 6 months and 18 months of age? 2. Is there evidence that breastfeeding influences tem- perament development or that difficult temperament in- fluences the continuation of breastfeeding? The conceptual model in Figure 1 shows the cross- lagged pathways representing these research questions. They are the cross-lagged pathways e and f, pointing from breastfeeding at age 6 months to difficult temperament at age 18 months, and from difficult temperament at 6 months to breastfeeding at age 12 to 14 months, respec- tively. Structural equation modeling will allow estimation and comparison of the size of the cross-lagged effects. From the *Norwegian Institute of Public Health, Division of Mental Health, De- partment of Psychosomatics and Health Behavior, Oslo, Norway; and †Psycholog- ical Institute, University of Oslo, Norway. Received March, 2008; accepted July, 2008. Address for reprints: Susan Niegel, M.Sc., Division of Mental Health, Norwegian Institute of Public Health, Department of Psychosomatics and Health Behavior, Postbox 4404, Nydalen, 0403 Oslo, Norway; e-mail: susan.niegel@fhi.no. Copyright © 2008 Lippincott Williams & Wilkins Original Article 458 Difficult Temperament and Breastfeeding Journal of Developmental & Behavioral Pediatrics