Studies in Second Language Acquisition, 2019, page 1 of 25. doi:10.1017/S0272263119000184 Research Article THE CONTRIBUTION OF PSYCHOSOCIAL AND BIOGRAPHICAL VARIABLES TO HERITAGE LANGUAGE LEARNERSLINGUISTIC KNOWLEDGE OF SPANISH Julio Torres* University of California, Irvine Ricardo Estremera State University of New York, Albany Sherez Mohamed Florida State University Abstract Individual differences (IDs) largely contribute to success in adult second language attainment (e.g., ornyei, 2006). Heritage language (HL) studies have also explored the role of IDs, namely psychosocial variables, and biographical factors with an adult HL learner population. However, the specic contribution of these variables to HL learnersperformance on linguistic tests that differ in degree of explicitness and modality remains unknown. Therefore, the current study tested 103 adult HL learners of Spanish who completed a spoken elicited imitation task (EIT) and a written untimed grammaticality judgment task (UGJT) that elicited their knowledge of vulnerable morphosyntactic structures in HL bilingual acquisition. To investigate the contribution of individual learner factors on their performance, participants completed a few questionnaires. Mixed-effects regression models revealed that sequential bilingual status, willingness to communicate, generation and motivation contributed signicantly, but yet differentially to participantsperformance on grammatical and ungrammatical items of the EIT and UGJT. We would like to thank Nick Pandˇ za and Bianca Cung for their guidance with statistical analyses. We are also grateful for the helpful comments from the SSLA editors and anonymous reviewers that improved upon the earlier version of the manuscript. All remaining errors are our own. *Correspondence concerning this article should be addressed to Julio Torres, University of California, Irvine, Department of Spanish and Portuguese, 322 Humanities Hall, Irvine, CA 92697. E-mail: torresju@uci.edu Copyright © Cambridge University Press 2019 4"C7:8 AC8 (8C"D A9 )D8 4*4!4!8 4( ((%D+++64"C7:8AC:6AC8(8C"D((%D7AAC:2 .A+#!A4787 9CA" ((%D+++64"C7:8AC:6AC8 ,668DD %47 I (8 3 /C*#8 0C4C8D A#  14I  4(  D)86( (A (8