The affect heuristic is a concept which advanced primarily from the work of Fischoff et al. in the late 1970's. It essentially describes our tendency to "go with our gut". Specifically, the affect heuristic is a shortcut mostly used when making automatic decisions, whereby we rely heavily upon our emotional response (our "affect") during decision-making, rather than taking the time to consider all the risks and benefits of our decision. If we think positively of something, then any rapid cost-benefit analysis will focus primarily on potential benefits. Similarly, if our immediate emotional reaction to something is negative, we will place more emphasis on risks and disregard potential benefits.
Fischoff et al. found that tobacco, alcohol, and food additives are all perceived as high-risk and low-reward items while X-rays, vaccines and antibiotics are seen as low-risk and high-reward. We see the risks and rewards as being negatively correlated, when in fact they are distinct and markedly different. This result is because smoking, drunkenness and food additives trigger negative emotional responses, whereas the other activities trigger positive emotions.