Objective-To assess dose-response relations between severity of iron deficiency (ID) and infant social-emotional behavior.Study design-Cohort of 9-to 10-month-old African-American infants (n = 77 with final iron status classification). Infants were provided oral iron for 3 months. Social-emotional outcomes included mother and examiner ratings at 9 and 12 months and quantitative behavioral coding from videotape at 12 months. General linear model analyses tested for linear effects of iron status group (ordered from worst to best -iron-deficient anemic (IDA), non-anemic iron-deficient (NA ID), ironsufficient (IS)) and determined thresholds for effects.Results-There were significant (p < .05) linear effects of poorer iron status for shyness (increasing, maternal rating), orientation-engagement and soothability (decreasing, examiner ratings), and the following quantitatively-coded behaviors: positive affect (decreasing) and latencies to engage with the examiner (increasing) and move away from the examiner (decreasing). The threshold for all but one effect was ID with or without anemia vs. IS.Conclusions-Infant social-emotional behavior appears to be adversely affected by ID with or without anemia. ID without anemia is not detected by common screening procedures and is more widespread than IDA. Infant social-emotional behavior can profoundly influence the caregiving environment, with repercussions for overall development.The prevalence of iron deficiency anemia (IDA) has markedly declined in US infants in the last 30 years. However, poor, minority, and immigrant infants and toddlers remain at increased Corresponding author: Betsy Lozoff, M.D., Center for Human Growth and Development, 300 N Ingalls, University of Michigan, Ann Arbor, MI 48109-5406. Tel: 734-764-2443; Fax: 734-936-9288; E-mail: blozoff@umich.edu Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Though findings of rapid improvement in mental test scores have not been replicated, 4 socialemotional alterations are among the most consistent findings. Virtually every study that compared social-emotional behavior of IDA to non-anemic (NA) infants found them to be more wary, hesitant, solemn, unhappy, or closer to their mothers. 5 Four of 6 randomized trials of supplemental iron that assessed this domain showed affective benefits of iron (e.g., more positive affect, social interaction, etc.). 5 Notwithstanding the consistency of results, socialemotional effects have captured less attention than cognitive ones, but they could equally result from direct effects of ID on associated brain systems. Furthermore, there h...