Like the field of linguistics, the field of psychology has significantly influenced the study of SLA.In linguistics the emphasis is on the structure of linguistic products (i.e., grammar formation).In psychology the emphasis is on the actual processing mechanisms as well as on issues that affect those mechanism (e.g., working memory capacity).Since 1990s, SLA research draw heavilyfrom studies of cognitive and developmental psychology, which studies how humans learn (i.e., acquire new knowledge and solve problems)