This is terrible math: [...] con artists and sociopaths out there [...] 4% of the population [...] you are going to get screwed over by at least one out of 1 out of every 25 people [...]

On the positive side, we try to put con artists in jail, so that moves the odds a bit in your favor. But con artists and sociopaths continually have to find new people to exploit as they are discovered by and/or burn out the old ones. And the skilled ones can gain disproportionate control of resources, further increasing the odds you'll be dealing with them. As an example, 99.7% of my unfiltered mail is spam, not 4%.

I think you should almost always act as if you're assuming positive intent, because that's how you build good relationships. But in practice, you should always be keeping an eye out for signs you're dealing with one of the many exploiters of the world.