This file contains the log-transformed, regressed and standardized synaesthesia screening score in combination with the log-transformed, regressed and standardized score of autistic traits within the sub-domain of Repetitive Behavior, Restricted Interests and Attention to Detail, RRBI-D, together with zygosity information (1 = monozygotic, 2 = dizygotic). Each data row belongs to one twin pair, and the data have been modified slightly by adding a small random noise for data protection purposes. The twin order is random, and variables ending with a one belong to one twin, and variables ending with a 2 to the other twin.