Between 1984 and 2009, the median net worth of a member of the House more than doubled, according to the analysis of financial disclosures, from $280,000 to $725,000 in inflation-adjusted 2009 dollars, excluding home ­equity.

Over the same period, the wealth of an American family has declined slightly, with the comparable median figure sliding from $20,600 to $20,500, according to the Panel Study of Income Dynamics from the University of Michigan…

Not only has the median wealth increased, but the proportion of representatives who have little besides a home has shrunk. In 1984, one in five House members had zero or negative net worth excluding home equity, according to the disclosures; by 2009, that number had dropped to one in 12.

Another possible reason for the growing wealth of Congress is that running a campaign has become much, much more expensive, making it more likely that wealthy people, who can donate substantially to their own campaigns, gain office.