Recent approaches to the "that"-trace phenomenon in English include syntactic analyses based on the principle of Anti-locality and a sentence production analysis based on the Principle of End Weight. These analyses have many similarities, but they differ in their predictions for second language (L2) speakers. In an Anti-locality analysis, we expect L2 speakers to show a pattern very similar to first language (L1) speakers, with substantial degradation in acceptability for extraction of a subject from an embedded clause with "that." In the Principle of End Weight analysis, we expect L2 speakers to display this same subject extraction degradation whether or not the embedded clause has "that." A sentence acceptability experiment with L1 English speakers and two groups of L2 English speakers (L1 Korean and L1 Spanish) confirm the prediction of the Principle of End Weight analysis: the L1 speakers show degradation with subject extraction from a "that"-clause, while the L2 speakers do the same with clauses with and without "that." These results form an interesting contrast with studies of island effects, which have generally found substantial L1[approximately]L2 similarities, and show how L2 data can be used as evidence to decide between competing analyses of L1 phenomena.