Do identical twins have identical synaesthesia?
No. Alford (1918) documented two male twins with letter>colour synaesthesia, and who agreed on only 67% of their colours. A second twin study (Smilek et al., 2002) reports two genetically identical females, one of whom has synaesthesia (letter-to-colour) and one who does not. Women contain two-copies of the X chromosome, only one of which may be active, and which one is active can differ between twins who are otherwise genetically identical.
