Anxiety and intelligence
We can detect anxiety through high levels of cortisol, but we can also see if we can predict lower achievement in test-taking environments by administering a personality questionnaire and looking for a relationship between certain traits and scores on a test. The Big 5 trait neuroticism includes sub-factors relating to anxiety, such as worry and fear, so it should follow that those who score highly on this particular trait should score poorly when taking tests. Rather than measure cortisol, we measure personality traits.
I'm therefore going to make a prediction, one that is actually supported by evidence. Of course, all claims should be open to stringent analysis and criticism, and the claim I am about to make is no exception. I can prove the claim to be statistically true, in that the data supports the hypothesis.
Prediction: Anxious people are less intelligent than less-anxious people.
I think this is a rather dramatic prediction, one that perhaps needs a little clarification, not to mention a great deal of discussion. There are also some terms that need defining if we're going to fully comprehend the wide ranging implications of such a bold claim.
First of all I think I need to clarity the term anxiety. This is relatively easy because I'm writing in Big 5 language. What I mean by anxiety is the trait that we associate with neuroticism. The second term we need to define is intelligence. This is also pretty straightforward because the intelligence we need to refer to must be measurable (or quantifiable, if we're getting technical). We therefore need someone to do something that will yield a number that we can then call intelligence, or Intelligence Quotient. How about IQ? It's short, it's snappy and serves our purpose well. We can measure IQ using a vast number of tests that all correlate with each other, that is, if I were to complete, say, the Stanford-Binet test and then Raven's Progressive Matrices (two common tests of IQ), I should end up with the same result on both. A score of 100 would denote an average IQ, anything over about 145 would nudge me over to the side of genius. In reality, I have a fair to middling IQ, a little above average but I'm not in the market to become the next Einstein or Hawking (although neither ever took an IQ test as far as we know). Once we have a result for IQ, all we need is a result for personality, then we can compare IQ components to personality components and see if there is any kind of relationship, or correlation.
What studies conducted in this way tend to find is that certain traits either have a positive relationship, a negative relationship or no relationship with IQ. Generally speaking, people who score highly on openness also score higher on measures of IQ than people who score lower on openness. This would certainly make sense, because openness is associated with factors such interest and curiosity and the desire to seek out new things. Curiously, even though conscientiousness is positively related with academic achievement, it correlates slightly negatively with IQ. Extraversion is the most problematic, with studies finding both a positive and negative relationship. Neuroticism, on the other hand, is consistently associated with lower IQ scores (Ackerman & Heggestad, 1997).
And there you have it, proof positive that anxious people are less intelligent. I suspect you might not be too convinced about my somewhat cruel conclusion. Hopefully, you are right now attempting to formulate alternative possibilities, after all, we have a correlation, but do we trust the causation? Why should anxious people be less intelligent than less anxious people? And, of course, you'd be right to think that there must be some other conclusion we could draw from this. We know that anxiety raises levels of cortisol and that cortisol can impair cognitive function, so it would be more likely that the anxiety is at least partly responsible for the low IQ score.
The problem we have, however, is that studies of this kind relate to trait rather than state anxiety, that is, there is something inherent in the person that has revealed a propensity towards anxiety. However, we do know that trait anxiety is a good predictor of state anxiety, which means that we can identify students who might be more prone to it. We also know that prolonged exposure to anxiety can lead to further cognitive disruption, greater susceptibility to infection and lower levels of psychological wellbeing. Stressors are cumulative, so by the time exam season begins, some students will have spent some time in a state of anxiety at the anticipation of exams, along with revision sessions, fear appeals, parental pressure and so on.