I thought maybe the kid got sick? But it could've been autism, I don't know. But I'm just stating what I've learned working in medical records. And yeah it can happen. Severe concussions, fever-induced Encephalopathy, near-drowning incidents where there was brain damage due to lack of oxygen, etc., can cause it, too. It's damage to small sections of the frontal lobe.

Autism is genetic. People are born with it or they're not, but there are various degrees to which it can manifest. And you're right, they DO have emotions, but it can be difficult for individuals born with the condition to show it. The problem with Autism is that they tend to lack the sensory filters that people without autism have. It's basically sensory overload.

Rather than the brain being able to block out excess from their senses, primarily hearing and bright lights, but sense of touch can also be overwhelmed, which is why those with autism often prefer touching softer objects, those with autism often have trouble blocking excess sensations.

This is why loud noises, the wrong feel of something, overly bright lights, and so forth, can overwhelm the individual(s) quickly causing a stress reaction. Simply put, people with autism tend to get a lot more information from their senses than those without it. And in a world as chaotic as our's, that's a problem.