Children's difficulty with passives is well known in various languages (Maratsos et al., 1985, Gordon & Chafetz, 1990, and Fox & Grodzinsky, 1998 for English; Pierce, 1992 for Spanish; Sano, 2000 for Japanese; Bartke, 2004 for German; Terzi & Wexler, 2002 for Greek, among others), but the source of difficulty is still an open question. There are two competing accounts for the late development of passives, proposed for English but extendable to other languages. One is Borer and Wexler's (1987) A-chain Maturation Hypothesis, which says children cannot handle A-chains (i.e. movement from object to subject positions) until a certain age of maturation. Due to this inability, children adopt an alternative structure that does not involve an A-chain, namely that of adjectival passives. The other is Fox and Grodzinsky's (1998) Theta Transmission Account. They claim that children's difficulty has nothing to do with A-chains, but with theta transmission. Children cannot transmit the external theta-role to the by-phrase in passives, and they come to interpret the by-phrase by default Agent assignment.
One question arising from the above discussion is what the two hypotheses predict for languages whose passive formation does not involve an A-chain or theta transmission; in other words, for languages like Korean. If children have difficulty either with A-chains or with theta transmission, it is predicted that passives in languages that have neither should be easier to acquire than passives in languages that have both.