The fundamental difference hypothesis (FDH), as formulated by Bley-Vroman (1990), claims that SLA tends to be nonconvergent because domain-specific linguistic mechanisms available in early childhood cannot be used for language learning in adulthood: Instead, second language (L2) learners deploy domain-general problem solving skills. I claim that nonconvergence is also true of some cases of unbalanced early bilingualism, when the target language was acquired in childhood, suggesting that nontarget attainment in these cases may have different roots--namely, inefficient learning mechanisms in L2 learners but insufficient input in early bilinguals. The FDH then predicts that early bilinguals should still be more successful at attaining nativelike knowledge than the L2 learners due to their early age of acquisition. This review article examines this prediction in light of recent studies of postpuberty L2 learners and unbalanced early bilinguals with nonconvergent knowledge of their first language. I show that the incidence of nativelike achievement is higher in the early bilingual groups than in the L2 groups and that differential performance by the two populations on different tasks lends some support to the processing claims of the FDH.