Many people think that complex thinkers at the peak of their field become that way after years of accrued mastery or development of expertise, but complex thinkers, or thinkers who may succeed in complex reasoning tasks and environments are often very different from experts who have mastered (whether through skill, or knowledge, or both) finite intellectual or artistic domains.
Complex thinkers tend to be transdisciplinary - and may be so from the get-go (i.e. childhood). The reason for this may be that their thinking is more pattern-related and iterative rather than logically related in a casual chain.
The funny thing is, there are budding complex thinkers that we sometimes see as children. They are often identified as bright and having unusual questions, but may not shoot to the top of their class because they seem to be dabbling or in no particular hurry toward mastery. These students may blossom in the right environments or with the right stimuli - complex game playing scenarios, the medical ICU, comparative history scenarios, business innovation, or financial markets. In our experience, this is not uncommon among gifted dyslexic families.
In one model of complex causal reasoning below (determining plausability of a scientific relationship), an interesting area of the brain, the parahippocampal gyrus, appeared to be particularly important. This was the same area of the brain (in a different fMRI view) that researchers had previously found to be important for story-based or contextual memory.
It's these folks you turn to when big decisions need to be made under complex and uncertain conditions. But you may be surprised - sometimes these gifts are only discovered in a young person when he or she is given a great challenge.
fMRI and Complex Causal Thinking
Context-Dependent Episodic Memory and fMRI
Eide Neurolearning Blog: Switch! - Cross-Disciplinary Learning