Although aggregate household wealth has risen in part as a result of the stock market boom over the 1990s, many individual households have accumulated little, if any, wealth.
Although few households have accumulated wealth, those who have are billionaires. 
mnli hypothesis: Although few households have accumulated wealth, those who have are billionaires.  premise: Although aggregate household wealth has risen in part as a result of the stock market boom over the 1990s, many individual households have accumulated little, if any, wealth.