American men in their 30s are earning less than their dads did, challenging a long-held belief that each generation will be better off than the one that preceded it, according to a new study published Friday.

The report, the first in an ongoing 18-month study on economic mobility in the United States, also revealed that the income growth of the median American household is declining.

The study was produced by a handful of politically diverse think tanks including the Pew Charitable Trusts, the American Enterprise Institute, the Brookings Institute, the Heritage Foundation and the Urban Institute. It looked at income levels of American men in their 30s, which can be a good indicator of lifetime income.