The research found that polygamy, widow inheritance, multiple female partners, and extramarital relationships -- in the past viewed as important for keeping society together -- increased vulnerability to HIV-AIDS.
"If one sexual partner in such sexual networks is HIV positive and sex is unprotected, the practice becomes an important driver of the pandemic," said the UN Development Programme (UNDP)'s Swaziland Human Development Report for 2008.
Several studies had identified polygamy as a negative influence on the spread of HIV but "a defensive attitude has been maintained by the cultural gate-keepers" to preserve the practice, the study said.
Swaziland's absolute monarch King Mwasti III has thirteen wives and polygamy is widely practised in the tiny kingdom, but the UN study hints it might be on the decline.
The impoverished mountainous kingdom has been particularly badly hit by southern Africa's AIDS pandemic, with close to 40 percent of the adult population affected by the virus.
The UNDP's report found that multiple sexual partners, the loss of virginity at a young age, high levels of inter-generational sex and inconsistent condom use were the pandemic's main drivers.
Also contributing were gender inequality, sexual violence, a high prevalence of STIs, low levels of male circumcision and the cultural norms.