How much we trust peo­ple influ­ences much more than just our inter­per­sonal rela­tion­ships and can even cost us a con­sid­er­able amount of finan­cial harm.

The study con­clud­ing this (pdf) sug­gests that too much or too lit­tle trust has a finan­cial cost equiv­a­lent to that of not attend­ing uni­ver­sity and shows that if we trust too much we assume too much social risk, but trust too lit­tle and we give up poten­tially prof­itable opportunities:

Highly trust­wor­thy indi­vid­u­als think oth­ers are like them and tend to form beliefs that are too opti­mistic, caus­ing them to assume too much social risk, to be cheated more often and ulti­mately per­form less well than those who hap­pen to have a trust­wor­thi­ness level close to the mean of the pop­u­la­tion. On the other hand, the low-trustworthiness types form beliefs that are too con­ser­v­a­tive and thereby avoid being cheated, but give up prof­itable oppor­tu­ni­ties too often and, consequently, under-perform. Our esti­mates imply that the cost of either exces­sive or too lit­tle trust is com­pa­ra­ble to the income lost by fore­go­ing col­lege. Fur­ther­more, we find that peo­ple who trust more are cheated more often by banks as well as when pur­chas­ing goods sec­ond hand, when rely­ing on the ser­vices of a plumber or a mechanic and when buy­ing food.

via Tim Har­ford