The Gottmans note that negativity bias has an evolutionary purpose but does little good in long-term relationships:

When we observe our partners (and, in fact, when we observe our life in general), we have the tendency to notice the negative instead of the positive—the negative leaps out like a flashing neon sign […] because of some evolutionary aspects of how the human brain works, we scan for problems by default, because solving them will help us survive (44).

This tendency helped our hunter-gatherer ancestors scope out predators in the wild and thus adequately prepare to escape them. As our evolutionary traits have not caught up with our mostly safer modern lifestyle, we are still wired to perceive danger more than positive attributes. However, science shows that we are mostly unaware of being occluded by this bias, even though we have become entrenched in the habit of conditioning ourselves to look for problems—which we then inevitably find.

Although negativity bias may have had a function in helping us survive predators, it is deeply destructive in romantic relationships, as criticism of what our partners are doing and contempt for our distorted view of their characters get in the way of our ability to bond.