Review the item:
When Matt shaved his head, his mother worried that he had joined a
cult the real reason for going bald, however,
was that Matt could get more attention and sympathy from girls who thought
he was sick with a dread disease.
A comma splice occurs when two complete sentences are incorrectly joined with just a comma. In the example above, no such comma exists. His mother worried that he had joined a cult is one sentence. The real reason for going bald ... begins the second sentence.
Because only empty space exists between the two, you should call this error a fused sentence.
To fix the problem above, you could put a period after cult and begin the with a capital T.