This entry briefly covers the history, demographics, research, clinical practice, diversity, debates, and trends surrounding marriage and domestic partnership in the United States. Who marries and why, when, and at what rate people marry is covered, as are some of the statistics behind alternatives to marriage, such as cohabitation, domestic partnership, and civil unions. It is beyond the scope of this entry to discuss in detail relationship dissolution and divorce, although information is provided insomuch as it relates to marriage and domestic partnership. The ability to form close relationships with others is a crucial component of life span development. In fact, an inability to do so may be considered partial criteria for some types of mental disorders (American Psychiatric Association, 2000). Psychologist Erik Erikson (1980) theorized that young adults must master intimacy over isolation if they are to move successfully through his proposed stages of psychosocial development. Apart from these theoretical obligations, much of global society sanctions the forming of close relationships that it deems appropriate. Proms, engagements, weddings, and anniversary celebrations serve to socially reinforce (usually heterosexual) couplings and the norms surrounding acceptable relationships. Marriage is the legal, and most often consensual, partnering of two persons of either sex. Domestic partnership can refer to any unrelated persons 18 years of age or older living together for a minimum specified period of time (for example, one year) and in a financially interdependent relationship. Both unmarried heterosexual couples and same-sex couples can apply for domestic partner status in those jurisdictions, companies, and institutions that recognize it. However, such distinction still falls short of the 1,138 federal benefits and protections afforded to legally married couples (U.S. General Accounting Office, 1997, 2004). For example, access to a partner's Social Security benefits, Medicaid and Medicare benefits, and veterans' pensions, and the exemption from gift and estate tax liabilities, are just a few of the laws mentioned in the U.S. Code that are affected by marital status. Only marriage offers couples such entitlements; civil unions, a proposed substitute for same-sex marriage and available in only a handful of states, afford no federal benefits and protections.