Complex sentence: If their feelings towards an activity is positive, then people are more likely to judge the risks as low and the benefits high whereas if their feelings towards an activity is negative, they are more likely perceive the risks as high and benefits low.
Split into simpler sentences: If their feelings towards an activity are positive, then people are more likely to judge the risks as low and the benefits high. On the other hand, if their feelings towards an activity are negative, they are more likely perceive the risks as high and benefits low.