Why is a person so easily deceived?

Talbot Simms　in ‘Forever’ by Jeffery Deaver is an unusual cop. He’s a statistician with the Westbrook County Sheriff’s Department. When wealthy county married couples begin killing themselves one after another, Simms begins to believe that there is something more at play. In either case, the husband was suffering from an incurable disease, but the wife was healthy, and the husband and the wife loved each other. The two couples with the same circumstances killed themselves one after another. Simmons thinks that from a statistical point of view, the probability of such things happening is quite close to zero, and begins to investigate what is behind it. Wonderfully good reading, and it is extremely amusing. And it is greatly ascribed to the uniqueness of the main character Talbot Simms.
As another instance comparable to this detective in its uniqueness, I can only think of Inspector Grant in ‘The Daughter of Time’ by Josephine Tey.
But to be perfectly candid with you, what I