IQ and Effective decision-making are independent variables

From “Dysrationalia” Among University Students: The Role of Cognitive Abilities, Different Aspects of Rational Thought and Self-Control in Explaining Epistemically Suspect Beliefs by Nikola Erceg, Zvonimir Galić, and Andreja Bubić.

From the Abstract:

The aim of the study was to investigate the role that cognitive abilities, rational thinking abilities, cognitive styles and self-control play in explaining the endorsement of epistemically suspect beliefs among university students. A total of 159 students participated in the study. We found that different aspects of rational thought (i.e. rational thinking abilities and cognitive styles) and self-control, but not intelligence, significantly predicted the endorsement of epistemically suspect beliefs. Based on these findings, it may be suggested that intelligence and rational thinking, although related, represent two fundamentally different constructs. Thus, deviations from rational thinking could be well described by the term “dysrationalia”, meaning the inability to think rationally despite having adequate intelligence. We discuss the implications of the results, as well as some drawbacks of the study.

Way too small a sample, and definitely an unrepresentative sample (students only). But intriguing research on a known, though not well quantified condition.

I know lots of people who are exceptionally bright and routinely make many bad decisions, sometimes owing lack of self-control, sometimes ideological obsession, sometimes other issues.

At the same time there are plenty of people of average IQ who do exceptionally well. Again, personality traits (self-control, goal orientation, etc.) combined with positive ideological or belief systems. The Millionaire Next Door by Thomas J. Stanley laid some of the quantification groundwork for this.