It's probably not surprising that the inhabitants of poorer countries tend to say, when asked in surveys, that they are not particularly happy (as compared to survey respondents in well-off countries).
And, over many years of research, it's been confirmed that, when surveyed, the population of countries with high(er) Gross Domestic Products (GDPs) tend in general to say they're 'happier' than those in low GDP countries.
But, in 1974, US-based professor of economics Richard Easterlin suggested there might be an associated paradox. According to the professor's research, if a country's GDP rises over time, there's no corresponding increase in the happiness of the population.
Put formally, the Easterlin Paradox states that :
"At a point in time happiness varies directly with income, both among and within nations, but over time happiness does not trend upward in correspondence with income growth." ref.
The original dataset was purely US based, but now similar results have been found across a wide range of countries.
Various explanations have been offered as a way of explaining the paradox :
Other research groups assert that the paradox isn't real - either because of claims that the original research wasn't robust enough, or because in some countries people do report they're happier over time - see Wikipedia
Prof. Easterlin, however, has rebutted such assertions in his 2020 IZA Institute of Labor Economics working paper The Easterlin Paradox DP No. 13923
"Critics of the Paradox mistakenly present the positive relation of happiness to income in cross-section data or in short-term time fluctuations as contradicting the nil relation of long-term trends."
Also see :Happiness and ageingplugin-autotooltip__plain plugin-autotooltip_bigHappiness and ageing
In general, people tend to be 'happiest' (or at least say they are) between the ages of 60 - 75. The effect, which various research groups have now verified in more than 70 countries, has yet to be explained.
Other researchers say the effect is too weak to be regarded as significant :