Objectives This study investigates the psychological language distance regarding types of English idioms among Korean undergraduate students to elucidate the mechanisms of language transfer, hypothesizing that the perception of semantic similarity between Korean and English idioms leads to positive transfer. Methods A total of 82 undergraduate students in a metropolitan city participated in a questionnaire study that categorized English idioms based on semantic and formal similarity between Korean and English. The validity of findings regarding language transfer is enhanced when data on psychological perception, reflecting the process of language transfer, corroborates comprehension data, indicating the outcome of language transfer. In the questionnaire, therefore, the participants' psychological perceptions of the ease and difficulty of English idioms were measured using a Likert scale, and their comprehension of English idioms was assessed using a multiple-choice format. In addition, since idioms with prior knowledge are automatically processed and language transfer does not work, participants were asked to mark the items that they had prior knowledge of to exclude them from the data analysis. Results Korean students' perceived ease with English idioms was in the order of cognate idioms > pragmatic idioms > L2-only idioms > false friends idioms, while the accuracy rates of idiom comprehension were ranked as cognate idioms > pragmatic idioms > false friends idioms > L2-only idioms. The results showed that semantically similar idioms (i.e., cognate idioms, pragmatic idioms) were perceived as easier and had higher accuracy rates compared to dissimilar idioms (i.e., L2-only idioms, false friends idioms), supporting the Contrastive Analysis Hypothesis concerning transfer of meaning. However, psychological perception and accuracy rate data for dissimilar idioms produced conflicting results regarding psychological language distance. Nevertheless, pragmatic idioms with similar meanings but different forms between Korean and English were perceived as easier and had higher accuracy than false friends idioms with different meanings but similar forms. This suggests that the perception of cross-linguistic semantic similarity, rather than formal similarity, triggers positive transfer. Conclusions This study confirms the Psychotypology Hypothesis proposed by Kellerman (1983), which posits that perceived cross-linguistic similarity leads to language transfer, as supported by corroborative evidence from psychological perception and accuracy rate data. However, conflicting findings within dissimilar idioms suggest the need for follow-up studies employing various methods to measure psychological language distance, thereby validating the reality of language transfer more robustly.