The proportion of income which a family can pay for shelter varies according to the country and the times. The disparity between what the lower income groups can pay and the rent required to acquire a building and amortize its cost, has generally widened with the years because of technological lag, rising costs of materials, labour shortages and building laws which impose more costly standards. When wages have risen, so have shelter costs, and the gap has continued unbridged. Subsidized public housing projects intended for the poor are often out of their reach and captured by middle- or high-income households.