Wiser Words Were Ne'er Spoken

From Thomas J. Stanley, Ph.D.:

America is often referred to as the land of the free. But most people in this country are not really free. They are tied to debt and a treadmill existence in terms of earning a living. At this moment, our federal government has promised future social benefits in excess of $50 trillion. That figure is approximately the same amount of the total personal wealth held by Americans. In the future, it is very likely that the government will not be able to provide the promised social benefits to our seniors. The typical household in the United States has a net worth of just over $90,000.

Some people joke that by the time an average American man is 35 years old, he's a virtual slave. His bank owns his house; his employer owns his money and health insurance; his wife and kids own his time (which always reminds me that I should marry wisely); and the government owns the first three months of his work.

I am surprised that the typical household has a net worth of just over 90K. At the same time, these kinds of financial statistics are notoriously difficult to calculate. If I have 100K and I take out a mortgage to buy a 200K house, then doesn't my net worth go from 100K to negative 100K overnight?