The authors analysed data from the 2007 Adult Psychiatric Morbidity Survey in England. The participants were adults aged 16 years or over, living in private households in 2007. Data from 6870 participants were included in the study...Happiness is significantly associated with IQ. Those in the lowest IQ range (70–99) reported the lowest levels of happiness compared with the highest IQ group (120–129). Mediation analysis using the continuous IQ variable found dependency in activities of daily living, income, health and neurotic symptoms were strong mediators of the relationship, as they reduced the association between happiness and IQ by 50 %

"The relationship between happiness and intelligent quotient: the contribution of socio-economic and clinical factors", Ali et al 2012; effect is weakened once you take into account all the relevant variables but does sort of still exist.