Final answer:
The pattern of population growth in the southern and western U.S. and decline in the northeast and Midwest can primarily be attributed to economic decline which has influenced migration patterns, with regions like the Sunbelt witnessing growth due to job opportunities.
Step-by-step explanation:
While the southern and western regions of the U.S. are experiencing population growth, the northeast and Midwest are losing population. The best explanation for this pattern is economic decline. Migration patterns in the United States have shown significant rural-to-urban shifts, and intermigration regions have changed due to various demographic shifts. Economic decline has particularly played a role in the movement of populations. In the latter half of the 20th century, as industries closed and wages declined, Americans migrated in search of work. This led to population growth mainly in the South and the Southwest, regions known as the Sunbelt. Additionally, with the aging population and a decline in birth rates, regions with shrinking youth populations need to attract labor, leading to increased immigration in some areas and population decline in others.