This attributes "Women hold 65% of the country's wealth", to Fortune Magazine, but we cannot confirm the cite.
A UK study says, "women currently own 48% of the nation's personal wealth" and increasing, but Michael says the UK may be different from the USA.
He also found this: "Women own more than 47% of the stocks (Source: Peter Hart and NASD and the Investment Institute)", and notes that 47% is less than 50%.
Finally, he sends this study:
According to estate tax returns data, in 1925 one quarter of the wealthiest 0.01 percent were women, Figures 1a; and 1b.2 This fraction rose rapidly through WWII and then more slowly to peak in 1969, when women neared parity with men. Since then, there has been a marked decline. By 2000, women's share had fallen to one-third -- this despite the increased economic emancipation of women commonly observed for the labor market.and concedes that "this talks about percent of women among the wealthiest and not total wealth".
I will have to research this further.
Update: Now Michael sends this:
The wealth of living individuals can be estimated from Federal estate tax return data using the estate multiplier technique. The fundamental assumption underlying this methodology is that estate tax returns filed for decedents who died in a particular year represent a random sample, designated by death, of the living population in that year. ...I could not find Figure B.
In 2001, there were an estimated 7.4 million adults, age 18 and older, with gross assets of $675,000 or more ...
There were nearly 4.0 million male top wealth holders in 2001, representing 53.7 percent of the top wealth holder population. These men had a combined net worth of $8.0 trillion, for an average net worth of nearly $2.0 million ...
There were over 3.4 million female top wealth holders, comprising 46.3 percent of the total. The combined net worth of these women was $5.8 trillion, while their average net worth was $1.71 million ...
While the average net worth of female wealth holders was more than 15 percent lower than that of males, averages can be very sensitive to outliers. When significant outliers exist, the median is often a better measure of the center of a distribution. The median net worth for male wealth holders was approximately $978,000, while the median value for females was nearly the same at almost $955,000. In fact, Figure B shows that the distribution of wealth for male and female wealth holders is very similar for most points, except for those above the 95th percentile, where male net worth values dominate. It is these larger values that account for the much larger difference in the average net worth between the sexes. While not included in Figure B, it is interesting to note that the left tail of the net worth distribution for males dips much lower (larger negative values) for points below the 1st percentile than for females.