This article centres on extreme altruism, rather than general good behaviour in the form of being charitable and generous to people we don't know, Lilygran. This research posits that a small group of people aren't necessarily recognising that they've done a good deed (so no personal pay-off for them to feel good about what they've done), but that they have a different sized amygdala which governs their extreme altruistic behaviour, and likewise some psychopaths have an unusually small amygdala, governing their unempathic, callous behaviour, which doesn't register with them, either.
I wonder whether it's all down to a tiny part of the brain - I understand how criminal psychopaths are assessed and generally deemed to be untreatable, and that many more psychopaths lead lawful lives but treat others badly, but the notion that the anti-psychopath has an urge to do extremely altruistic deeds with no benefit to themselves, even a feel-good factor, is very interesting. Is there anyone reading this who knows an individual like this, who is dismissive of the idea that they've done a fantastic thing for a complete stranger?