The principal reason for declining natural increase rates in less developed countries today is

Declining crude birth rates.

Explanation:

Birth rate may be defined as the number of births takes place per year per thousand of the population. The birth rate and death rate may change the population dynamics.

The developing countries shows the expanding population graph as they have more number of people of the reproductive age. The advancement in the medical field results in the decline birth rate that causes the decline of the nature resources.

Thus, the answer is declining crude birth rates.

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."

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