Well, profit is just one side of a trade made by two parties that that hasn't been fulfilled yet. An IOU, if you will.

It does seem pretty clear that all social behaviour is motivated by equitable exchange. We don't do things for rocks – they offer nothing in return. We do things for other people because they do give something back in return. What comes back in return need not be complex. Perhaps it's just a hug, smile, or the press of an arrow button. But there is always something.

I'm not sure profit itself is a motivator. If we agree that you will give me a smile when I crack a silly joke for you, it doesn't matter much when the smile occurs. If you give it to me right away there will be no profit, but I still got what I wanted. If you wait, I profit, but that just means I get the smile later. What's the difference?

Sometimes waiting is significant. If our deal is for you to feed me, I don't want the food when I'm not hungry. It is sensible to want to hold that profit until I am hungry, at which time then our transaction can be settled. Maybe you can say profit is a motivator in that circumstance, I guess. It's really just a tool, though. The actual motivation comes from the trade – being able to get what someone else has.

Perhaps even more significant, it turns out that we get old and start to lose capacity to offer a lot of things to other people in our elderly state. As such, it is sensible to collect those promises of fulfillment (i.e. profit) in our younger days in order to get back things we need later in life when we need them. I think it is fair to say that this is a necessity. Trading something for food in your 20s to eat in your 90s doesn't work so well. It will have turned to dirt long before you get there. Trading something in your 20s for a promise to deliver food in your 90s works much, much better.

But, again, being able to delay fulfillment of a trade until later is a tool. I am not sure it is the motivation itself.