I have no idea what this study says. However, it may be talking about the loss of private wealth due to drops in the value of real estate, stocks, etc. This has certainly happened, but I doubt it adds up to 30% of the total wealth of the country.
It should also be noted that the decline is from the artificially inflated prices of a bubble. IOW, the values of 2007 were not real, they were merely an illusion.
Tell that to my mortgage company. They still want payment for the fully inflated, near-peak purchase price of my home.
Lawyers are scum.
Cheers!