The emotional and rational aspects of the brain are separate, almost competing. The first thing you learn in combat training that's worth a damn is that being hot on emotions and/or adrenaline makes you illogical. People do stuff they later said was incredibly stupid. This is also said by many perps of domestic violence where anger kept building until the urge to attack was uncontrollable. Logic just wasn't there anymore because that's not where logic comes from.

We're taught to recognize we won't have as much self-control, train ourselves to avoid letting emotions control us in an encounter, spend times when we're calm to prepare for likely situations with rehearsed reactions, and defined points when we stop attacks. The combination of these, esp with repetition, lets us handle a highly stressful and illogical situation with low risk of permanent consequences. An ability almost nobody in these classes started with.

Same is true in customer service, management, etc where you're dealing with stress, anger, and/or uncertainty. Same principles. The customer service people most of all since they deal all day with people saying or doing things that would piss them off. By not getting pissed, they keep customers, get paid, and keep their companies from loosing customers. So, the principle to avoid acting in anger where possible just keeps showing up with financial losses or jail time avoided many times over by following it.

Natural selection works in context. Any trait that is selected is selected in context, there is no absolute right or wrong in natural selection.

The fact that a trait has been selected says nothing of its intrinsic value or its utility in a different context. It certainly bears no moral value.

Surely, you would agree that the context we experience today is a little different from the time where selection pressure could favor anger?

I don't think the idea is that it makes you dumb your whole life. Just in the moment.

That matches my own personal observation at least.

Of course it only takes a moment to ruin your whole life. Or somebody else's.

> If it made you dumb we would not have evolved into the intelligent species we are today.

That is the whole argument of western civilization, that suppressing anger is what allows us to be an intelligent species.