The 20-item prosopagnosia index (PI20) was developed for assessing congenital prosopagnosia, which is characterized by severe facial recognition deficits in the absence of any obvious neurological deficit. We aimed to develop a Japanese version of the PI20 (PI20-J) scale and evaluate its validity and reliability. In study 1, we confirmed the internal consistency, test-retest reliability, and concurrent validity of the scale. In study 2, we examined the relationships between PI20-J score and facial recognition performance and found a moderate correlation between them. In study 3, we examined whether the PI20-J score is related specifically with facial recognition performance, or with general object recognition performance. We found that participants with a high PI20-J score showed weaker facial recognition performance than those with a low PI20-J score. In contrast, the object recognition performance did not depend on the score. Our results suggest that the PI20-J score is related specifically with facial recognition performance. We conclude that PI20-J is highly reliable and valid as a self-reported measure for congenital prosopagnosic traits.
