Intelligence and rationality are different
Intelligence may involve memory, problem-solving and verbal skill. Rationality requires using those abilities to evaluate evidence fairly and revise conclusions when necessary.
Reasoning often serves prior beliefs
People frequently begin with a preferred conclusion and then search for arguments that defend it. Greater intelligence can produce more sophisticated justification rather than correction.
Identity protects beliefs
A belief connected to religion, politics, profession or social group may feel like part of the self. Evidence against it can then be experienced as a personal or social threat.
Expertise may be narrow
A person can be highly skilled in one field while lacking knowledge in another. Success in one area may create unjustified confidence elsewhere.
Emotional needs influence judgement
Beliefs may offer hope, certainty, superiority, belonging or relief from fear. These benefits can make weak explanations difficult to surrender.
Good reasoning requires habits
Intellectual humility, source checking, exposure to criticism and willingness to change one's mind are practices. Intelligence alone does not guarantee them.
Evidence notes
A strong evaluation should examine whether a person applies the same evidential standards to favoured and unfavoured claims and whether they can state what would change their mind.
Ethical questions
Are we evaluating evidence or defending an identity?
Do we demand stronger proof from opposing views than from our own?
Can intelligence increase confidence without increasing accuracy?
Conclusion
Intelligent people can believe irrational things because intelligence is a tool, not a guarantee of fair judgement. Rationality depends upon how reasoning is used and whether correction is welcomed.