Since the 1990s, developments in technology and regulation have transformed the US mortgage market. Lenders have become more sophisticated both in terms of underwriting credit and financing it (Mian et al. 2010, Fostel and Geanakoplos 2012, Favara and Imbs 2015, Foote et al. 2018). These changes on the supply side contributed to changes in borrower characteristics and affected the stability of the mortgage market. For instance, before 2007 a significant portion of credit was issued to risky borrowers, which played an important role in causing the financial crisis (Mian and Sufi 2009).
A vast literature has documented changes in the mortgage market over the last few decades and the implications of those changes, but it has largely ignored one ubiquitous aspect, namely the presence of a co-borrower, i.e. an additional borrower responsible for the repayment of the mortgage.
https://voxeu.org/article/secular-decline-co-borrowers-mortgage-market-not-harmless