David Cameron’s #projectfear uses a well know psychological technique called loss aversion. Wiki:
In economics and decision theory, loss aversion refers to people’s tendency to strongly prefer avoiding losses to acquiring gains. Most studies suggest that losses are twice as powerful, psychologically, as gains. Loss aversion was first demonstrated by Amos Tversky and Daniel Kahneman.
This leads to risk aversion when people evaluate an outcome comprising similar gains and losses; since people prefer avoiding losses to making gains.
This is why politicians always say in advance they will “run a positive campaign” but then end up running a negative one – fear of downsides outsides potential benefits of upsides in the average person’s mind.
That’s also why entrepreneurs, inventors and explorers tend to be in the vanguard of the Leave campaign – they don’t allow irrational fear of small risks to keep them from trying for the big prize.