Okun's law..........it says that for every point that GDP growth is above 3%, the unemployment rate falls by 1/2 a percentage point. (For example a 4% rate of growth in GDP would lead to a reduction in unemployment from 4.5% to 4.0% over the course of a year).
it has held up pretty well over time (it was developed in the 60's) which is uncharacteristic of most macro theories