Speaking about concepts, it seems important to distinguish between altruism as a long-term character trait, and a short-term behavior. When someone's situation improves, they may feel more generous and help others (e.g. someone gets a raise, or wins a lottery); but people who don't help others or even actively exploit them can accumulate more resources (e.g. usurers, thieves).

Conditions... is probably also necessary to distinguish between long-term (e.g. social class, urban or rural, age, having children), and short-term (e.g. hungry, tired).

How do we measure altruistic behavior? Some people prefer to donate time, some prefer to donate money. Should we measure the donated money in absolute numbers, or as a fraction of income/wealth? (If a poor person and a rich person donate the same amount of money, should we consider them equally altruistic, or is the poor person more altruistic? If a busy person donates time are they similarly more altruistic than a non-busy person?)

Helping your ingroup creates an obligation to reciprocate in the future. Helping strangers does not have the same effect, but it still can serve as a costly signal of wealth. I suppose poor people prefer the former, and rich people prefer the latter.

EDIT: It might also make sense to distinguish what effects would altruism have in our evolutionary past, and what effects it has now. Some altruistic actions could be more "profitable" in the past. For example, if you help literally everyone, today it would mean mostly strangers, but in the past it would mean mostly members of your tribe. Or, without the rule of law, "I help you, so that you have an incentive not to kill me" is more important.

I think you might also find this study interesting:

This study looked at personality trait and personality disorder correlates of self-rated altruism. In two studies over 4,000 adult British managers completed a battery of tests including a ‘bright side’ personality trait measure (HPI); a ‘dark side’/disorders measure (HDS), and a measure of their Motives and Values which included Altruism. The two studies showed similar results revealing that those who were low on Adjustment (Neuroticism) but high on Interpersonal Sensitivity (Agreeableness), Prudence (Conscientiousness) and Inquisitiveness (Openness) were more likely to value Altruism and be motivated to commit altruistic acts which concerns helping others and creating an environment that places emphasis on customer service. Those more interested in “Getting Along” with others were more Altruistic than those more interested in “Getting Ahead” of others. Implications for the selection and management of altruistic people in a business are considered. Limitations and future directions of this research are also noted.