For talking about links between wealth and happiness, it matters greatly what absolute level of wealth you are talking about. Within first world countries, the link between between wealth and happiness is weak. But when comparing between countries that are rich versus on the brink of starvation, the correlation is one of the strongest known.

This may seem contradictory makes sense if you think of it in terms of Maslow's hierarchy of needs. If you are at the base level of meeting your physiological needs for food and shelter, wealth directly helps you meet those needs and makes them happier. If you can take care of that, the law and order that brings you safety is again correlated with being in a wealthier society. But beyond that, the correlation vanishes because wealth doesn't give you love, esteem or self-actualization.