As the variable input of production is increased, the average product could be either positive or negative.
This is because as more variable input (like labor) is added to a fixed input (like capital), the total product can increase at an increasing or decreasing rate. If the total product is increasing but at a slower rate, or if total product is decreasing, then the average product, which is calculated by dividing total product by the quantity of the variable input, could become negative if total production falls below the level necessary to keep the average above zero.