Final answer:
The principal reason for declining natural increase rates in less developed countries is economic factors, as increased economic security often leads to lowered birth rates following a transition period where death rates initially drop leading to immediate population growth.
Step-by-step explanation:
Judging by demographic patterns of recent years, we can surmise that the principal reason for declining natural increase rates in less developed countries today is economic factors. Numerous studies have demonstrated that as economic security increases, a demographic shift occurs and birth rates decline. This connection is evident across different societies and has been a consistent pattern. Economies that have seen improvements in public health measures, sanitation, and nutrition often experience a significant drop in death rates which, early in development, leads to a rise in population. However, as these countries develop further, economic development and factors such as better education for women and urbanization lead to a decrease in birth rates.
Moreover, as the structure of families and labor changes with development, individuals in less developed countries begin to move away from agriculture towards urban economies that offer different employment and educational opportunities. This shift typically results in smaller family sizes as the economic burden of raising many children becomes apparent. Hence, economic factors are the principal driving force behind the declining natural increase rates seen in less developed countries.