Looking for millionaires? Take a drive around Singapore's residential districts – more than one in six houses you see will be inhabited by one.

The Southeast Asian city-state has retained the honor of having the highest percentage of millionaires in the world, according to a new report from the Boston Consulting Group. Singapore had 188,000 millionaire households in 2011 – or slightly more than 17% of its resident households. Effectively, that equates to more than one in every six Singapore households having disposable private...