Actually, opinions vary among writers and editors on the usage or avoidance of the serial comma. In American English, the serial comma is standard usage in non-journalistic writing that follows the Chicago Manual of Style.

Journalists, however, usually follow the AP Stylebook, which advises against it. It is used less often in British English, where it is standard usage to leave it out.

So basically, there is no universal convention on the usage of a serial comma.