Final answer:
The pull factor of heading West to find gold was an economic factor, highlighted by the California Gold Rush in 1848, which was part of the American Westward expansion.
Step-by-step explanation:
The pull factor of heading West for the opportunity to find gold was primarily an economic factor. During the period of the American Westward expansion, especially the California Gold Rush starting in 1848, people were motivated by the prospect of wealth and the promise of a better economic future.
This mass migration led to significant demographic changes and was part of the larger dynamic of the United States' expansion which also featured various push factors including poverty, land shortages, and political unrest in other areas.