Literal understanding is much easier than irony or metaphor, although by age 7-9, children are well on their way to understanding both. Metaphorical meanings are usually understood before irony, which has greater demands on theory of mind or putting yourself into another person's perspective, and subtleties of language meaning and situational context.
In the figure below, the activation at the front of the brain in irony and metaphor (right hemispheric view) shows activated pathways seen to be important in other studies involving reflective thought.
It's the right hemisphere that's thought to be causing the greater difficulty that kids diagnosed with Aspergers have perceiving different word interpretations or associations with ambiguous narratives.
Metaphors are very valuable tools creative and analytical work of all kinds, whether science, history, law, negotiation, entrepreneurship, or self-understanding, as some of the links below attest. But misleading metaphors can also be obstructive or destructive to good thinking, as the bottom reference (a thesis) argues.
fMRI of irony and metaphor
Metaphors in Science
Metaphors at Purdue OWL
Teaching Metaphorical Thinking
ENL Blog: Metaphorical and Gifted Thinking
Analogical Thinking and Imagery in Problem Solving
Metaphors We Live By
Metaphors Bewitch, Analogies Illustrate, and Logic Fails - Controversies Over the Use of Metaphoric Reasoning
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