Complex sentence: Illegal immigration is overwhelmingly upward, from a poorer to a richer country., and one measurable factor is the "push-pull" incentive-the quality of life in the host country against the home country.
Split into simpler sentences: Illegal immigration is overwhelmingly upward, from a poorer to a richer country. One measurable factor is the "push-pull" incentive-the quality of life in the host country against the home country.