Suzanne Flynn and Wayne O'Neil Massachusetts Institute of Technology I. INTRODUCTION The theory of Universal Grammar (UG) as explicated e. g. in Chomsky, 1986, has led to explosive developments in the study of natural language as well as to significant advances in the study of first language (L I) acquisition. Most recently. the theory of UG has led to important theore tical and empirical advances in the field of adult second language (L2) acquisition as well. The principle impetus for this development can be traced to the work in linguistics which shifted the study "from behavior or the...