In response to new theoretical claims and inconclusive empirical findings regarding relative clauses in East Asian languages, this study examined the factors relevant to relative clause production by Korean heritage speakers. Gap position (subject vs. object), animacy (± animate), and the topicality of head nouns (± topicalization) were manipulated as experimental variables, and factors that appear to have been confounded in previous studies (e.g., context, proficiency) were controlled for or measured. Data were collected from Korean native and heritage speakers using an elicited production task. Group results failed to show a subject advantage, but individual analysis revealed that the effect of gap position varied for speakers at different levels of Korean proficiency. Results from the topicalized lead-in condition as well as error analysis revealed two different sources related to these patterns: Traditional asymmetry (i.e., subject advantage) appears to reflect the noun phrase accessibility hierarchy effect, whereas backward asymmetry (i.e., object advantage) seems to stem from word order strategy. Proficiency scores exhibited a modest correlation with the different individual response patterns. However, an animacy effect was not observed. The implications of these findings are discussed in terms of a general learning strategy, language universals based on informational prominence, the role of proficiency, and the effect of task demands.