in a tenancy in common (co-owners without survivorship), each co-owner is taxable on the income attributable to the taxpayer's share. Co-owners who are joint tenants (with survivorship) split income from their property according to their ownership interest.
Similarly, any gain (or loss) from sale of jointly-owned property is divided among the co-owners unless the joint ownership was created to save taxes on the sale, in which case the original owner is taxed on the full amount of the gain
this comes from RIA Federal Tax Handbook
so how is ownership interest determined? that's usually done based on the amount each taxpayer contributed.
