Kahneman divides our thinking into two subsystems: type 1 and type 2. Type 1 thinking is fast, intuitive, unconscious thought. Most everyday activities (like driving, talking, cleaning, etc.) make heavy use of the type 1 system. The type 2 system is slow, calculating, conscious thought. When you're doing a difficult math problem or thinking carefully about a philosophical problem, you're engaging the type 2 system. From Kahneman's perspective, the big difference between type 1 and type 2 thinking is that type 1 is fast and easy but very susceptible to bias, whereas type 2 is slow and requires conscious effort but is much more resistant to cognitive biases.
Traditionally, "intelligence" (as colloquially defined) has correlated with type 2 thinking. So, a reasonable conjecture would be that people who are better at type 2 thinking would use it more and, therefore be less vulnerable to bias. However, this research shows that even those who are very good at type 2 thinking (as measured by their SAT scores and NCS scores) are even more vulnerable to cognitive biases. This is a deeply counter-intuitive result. Why is it that people who have a greater capacity to overcome bias have a greater vulnerability to bias?