I think the relationship between income and the happiness of an individual is in line with the principle of diminishing marginal utility. Money can help to increase one's feel/feeling of happiness in almost every way that a person can be helped. Money can bring a person better life conditions and make most of the problems in life easier to solve. Money can even bring more feeling of value to the person by enabling/allowing them to help others. At this stage, the relationship between income and the happiness of an individual is a positive correlation, which means the more money a person has, a happier life a person can live. But if the curve goes further, a person who already has billions (of money) (you really need to say the currency here, billions of dollars etc, or is a billionaire, or is very wealthy, or just say ‘has billions’ as its clear you’re talking about money) , he /she is now pursuing the meaning of life and inner peace, the curve will go flat at this stage. Because at this stage, money can not bring answers to a person. Well done. A few corrections. Person is singular (one person, two people). So is curve.