Biological and Behavioral Factors Influence Group Differences In Prolactin Levels Among Breastfeeding Nepali Women JOY F. STALLINGS, 1 * CAROL M. WORTHMAN, 1 AND CATHERINE PANTER-BRICK 2 1 Laboratory for Comparative Human Biology, Department of Anthropology, Emory University, Atlanta, Georgia 30322 2 Department of Anthropology, Durham University, Durham DH1 3HN, United Kingdom ABSTRACT This study of two natural fertility Nepali groups, the Tamang and Kami, identifies biological and behavioral factors associated with population differences in fertility. Previous research had established that Tamang experience longer interbirth intervals than Kami despite simi- larly intense nursing practices, and bear considerably higher energy expen- diture due to workload. This cross-sectional study of 71 breastfeeding women includes prolactin determinations on three blood spot samples collected 5, 30, and 50 minutes following a timed nursing bout, and data on maternal age, BMI, menstrual status, previous birth interval, parity, infant age, nursing bout length, and durations of supplementation and postpartum amenorrhea. The findings show that Tamang breastfeeding mothers have higher average prolactin levels than Kami for as long as 22 months postpartum. Tamang mothers sustain average prolactin levels above those of nonpregnant, non- lactating women for nearly 2 years postpartum, whereas prolactin levels among Kami breastfeeding mothers are the same as this latter group after 1 year postpartum. Furthermore, the findings indicate that Tamang mothers have higher average prolactin levels regardless of maternal age, physical status (BMI, weight, or height), or infant age, and the rate of decline in prolactin from 5 to 50 minutes after suckling is significantly greater for Kami than Tamang. Since factors associated with prolactin levels differ by group, the findings also emphasize that populations vary not only in the strength of effects that proximate determinants have on fertility regulation, but also in the pathways by which they exert their effects (direct physiological versus indirect behavioral). Am. J. Hum. Biol. 10:191–210, 1998. © 1998 Wiley-Liss, Inc. The present report explores how cultur- ally mediated behavioral differences medi- ate group differences in fertility. A study of two rural Nepali communities, conducted in 1982–1983, probed the relationship be- tween breastfeeding and birth spacing (Panter-Brick, 1991). Factors evaluated for their role in influencing nursing mothers’ fertility were feed duration, interfeed inter- val, feed frequency, and total nursing time per day. Detailed observations of mothers and their infants supplied contextual expla- nations for effects of maternal subsistence activities on nursing patterns. The study identified fertility differences between two groups in the study population, the land- holding agropastoralist Tamang (interbirth interval  37.7 mo, S.D. 16.1) and land-poor blacksmith Kami (interbirth interval  29.4 mo, S.D. 11.7), and linked these to spe- cific cultural differences in nursing and working behavior. Fertility differences be- tween the two groups are related to both social and biological factors, as the Tamang Contract grant sponsors: Emory University Research Commit- tee and W.T. Grant Foundation. *Correspondence to: Joy F. Stallings, Department of Anthro- pology, Emory University, Atlanta, GA 30322. Received 8 July 1996; Accepted 13 April 1997 AMERICAN JOURNAL OF HUMAN BIOLOGY 10:191–210 (1998) © 1998 Wiley-Liss, Inc. PROD #733