“That Matthew is essentially a redaction of Mark is almost universally agreed. He borrows extensively from Mark (nearly the whole narrative), and frequently duplicates his material verbatim. Matthew then added a ridiculous Nativity Narrative (which no reasonable historian should regard as anything but fiction) and a brief but vague resurrection-appearance narrative (to fix what he may have regarded as the unsatisfying ending of Mark), which most historians also doubt is historical, and then revised the material in between, often altering or expanding on the stories Mark invented, occasionally inventing new ones and adding large sections attributing new teachings to Jesus.”
-- Richard Carrier, “On the Historicity of Jesus”