 Twin studies suggest that there are likely genetic causes of transsexuality, although the precise genes involved are not fully understood.One study published in the "International Journal of Transgenderism" found that 33% of identical twin pairs were both trans, compared to only 2.6% of non-identical twins who were raised in the same family at the same time, but were not genetically identical.Ray Blanchard created a taxonomy of male-to-female transsexualism that proposes two distinct etiologies for androphilic and gynephilic individuals that has become controversial, supported by J. Michael Bailey, Anne Lawrence, James Cantor and others, but opposed by Charles Allen Moser, Julia Serano, and the World Professional Association for Transgender Health.