Inequalities of wealth are significant and growing, both between countries and within both rich and poor countries. They are increasingly believed to reduce societal wellbeing.
When communities are very poor they are less able to afford reproductive health services. They are also more likely to want several children to support them in old age and to help generate income, though this can be counter-productive if their wellbeing is really limited by the amount of land or water available to grow crops.
Large differences in prosperity between countries drive people to migrate. However, large-scale migration can undermine the stability and therefore the sustainability of the destination country and deplete the country of origin of scarce skills.
In more prosperous countries most people have sufficient to meet their basic human needs. Once this has been achieved, further consumption increases wellbeing only at a diminishing rate. There is evidence to show that when people see others enjoying things they can't afford themselves it can make them unhappy, even if the things involved don't directly add anything tangible to their wellbeing. In a very unequal society, especially where the super-rich are conspicuously affluent, people will aspire to an unsustainable "celebrity lifestyle". This reduces happiness and increases the overall level of consumption when there simply are not the resources available for large numbers to live in such a manner.