An occupant detection system comprises first and second electric field sensors mounted in or on a seat back of a seat, and at least one seat weight sensor. The first electric field sensor provides a measure of the central/inboard capacitance of a first electrode located proximate to an upper central region of a seating surface of the seat back, and extending downwards towards a middle inboard region of the seating surface. The second electric field sensor provides a measure of outboard capacitance of a second electrode located proximate to the outboard side of the seating surface. If the central/inboard capacitance is less than an associated central/inboard capacitance threshold, and if the measure of seat weight is less than an associated weight threshold, then the side air bag inflator is disabled. Otherwise if the outboard capacitance is greater than an associated outboard capacitance threshold, then a side ratio SR=(W/M)n·(C1/C2) is calculated. If the side ratio is less than an associated side ratio threshold, then the side air bag inflator is disabled. Otherwise the.side air bag inflator is enabled for deployment responsive to a signal from the side-impact crash sensor.