Over the next few years, instead of answering questions, Matthew just (a)   them.

He focused intently on (a)   and other objects, yet had trouble looking his parents in the eye.

Eventually, when the boy was 5, an expert diagnosed Matthew with a brain disorder called (a)   .

Autism changes how the brain (a)   .

As a result, affected kids often have trouble (a)   with other people.

They may repeat over and (a)   the same behavior or action---rocking back and forth.

As a term, "autism" can refer to any of many different (but closely related) (a)   .

(a)   refer to the group as "autism spectrum disorders."

Indeed, researchers use the word "spectrum" because autism's symptoms can (a)   so much from one person to another.

"It is such a broad-spectrum disorder: You can have a (a)   bit of autism or a lot of autism," Laura Shumaker says.

"You can be a (a)   with autism but have great difficulty interacting with people."

"You can be non-verbal and very bright, or just like Matthew: He's a nice, earnest guy who's very (a)   . Everyone's different."

Even though autism diagnoses are becoming more (a)   , no one knows exactly why.

Many kids with autism (a)   to do the things that might come naturally to others their age.

Children with autism may want to join in group play but may not know how to (a)   that interest in a way that others recognize.

More than 20 years after Matthew's diagnosis, researchers still don't know what causes (a)   .

That's important because other research shows that early treatment can make a big difference in (a)   autism's impacts.

Autism can be socially (a)  

"Imagine waking up in the morning and wanting friends but just not being able to (a)   with them."

Kids with autism may have a hard time showing (a)   .

That makes it harder for others to know if they're (a)   or sad.

Similarly, children with autism have a hard time reading facial (a)   . So they may not be able to figure out how others are feeling.

"Certainly, they enjoy things and are happy and get frustrated and are sad, just like all of us, " says Kathy Angkustsiri, a doctor and (a)   expert.

"But they may not show it the same way." With autism impairing someone's (a)   to show or read emotions, "You can see how that can get you into a lot of trouble as you get older and have to hold a job."

People with autism "don't have the same ability to pick out the most important (a)   cues," Angkustsiri says.

"It's like the volume is just turned up all the way." And if everything is at the same (a)   , she says, "you can't sense the things that should stand out more."

Some scientists like to say that the brain is " (a)   differently" in people with autism.

That means they often see, hear, and feel the (a)   in ways that the rest of us don't.

Indeed, some children with autism can be extremely (a)   to loud noises, bright lights and even textures of food.

They may (a)   to these sensations, yet still not respond when a parent calls their name.

Matthew Shumaker, now 27, was diagnosed as a child with an autism (a)   disorder.

Over the years he has learned to interact well with other people. He now runs a (a)   business.

When Matthew was young, Laura Shumaker says he heard every noise. What's more, "It was like a very loud (a)   all the time." He'd be sitting in school trying to complete a workbook assignment. But "He couldn't get the work done," she says because every noise was so distracting and bothersome.

When he was in second grade, someone pulled a fire alarm at school. For the next three months, Matthew refused to take his (a)   off of his ears.

It's one example of the exaggerated reactions that people with autism may have to events that (a)   them.

Because people with autism can react to events and (a)   differently, many myths have cropped up about the disorder, Laura Shumaker says.

Many people, for instance, believe that children with autism are in their own (a)   and don't like to be touched.

"Nothing could be farther from the truth for my son," she says. "He's hungry for (a)   ."

Scientists don't know the exact cause of autism. But they are finding more and more clues that suggest that the varied brain disorders seen across the autism spectrum are due in part to the (a)   that babies inherit from their parents.

Those (a)   ultimately influence what an organism looks like and how it behaves.

If a child has autism, researchers know that the child's (a)   are more likely to have it too.

So far, scientists have found hundreds of genes that might be linked to autism. But genes (a)   can't fully explain why some kids develop autism and others don't.

"The reality is that we don't fully understand most of the causes of autism spectrum disorders," says Warren at Vanderbilt University. "The reality is that there are likely many, (a)   causes."

Some researchers think that differences in a baby's (a)   system---how well it fights off infections---might play a role.

Like electronic circuits, some (a)   of interconnected nerve cells relay thoughts, sensory information, and more throughout the brain.

In people with autism, some of these " (a)   " may not develop or connect normally.

This may make it hard for them to appropriately process (a)   , sounds or facial expressions.

Such research suggests that autism might begin before (a)   when this part of the brain begins organizing itself.

The new study also supports the idea that treating autism early in life may (a)   brain connections allowing the circuitry to form detours around damaged areas.

One thing that doesn't appear to trigger autism: childhood (a)   . Yet for years, many parents have heard rumors that these shots could be a cause.

In fact, there is no link between autism and vaccines, says Dr. Peter Jay Hotez, a pediatrician. "There's no (a)   that any of these vaccines are linked with autism," Hotez argues.

Autism leads to so many complicated changed in the brain that it couldn't be the result of giving (a)   a vaccine, he says. Plus, vaccines save lives.

Autism: a mental condition, present from early childhood, characterized by (a)   in communicating and forming relationships with other people and in using language and abstract concepts.