Which economic factor contributed most directly to the start of the great depression?

The economic factor that helped create the Great Depression the most is the allowance of people to buy stocks on a margin. Buying on the margin is simply borrowing money from the brokerage to buy the stock. This allows people to buy more stock than they can afford to. When they get lucky and it pays out all is good, but when the stocks they pick tank, they are left with far more debt than they can affor.

The law of supply and demand