Final answer:
The decline in U.S. adolescent pregnancy rates is largely attributed to later initiation of sexual activity and increased contraceptive use, with comprehensive sex education being favored by most Americans over abstinence-only programs for its effectiveness in reducing pregnancies and STIs.
Step-by-step explanation:
The factors that have contributed to the decline in adolescent pregnancy rates in the U.S. are multifaceted. While there are several potential influences, the evidence points to a few key factors such as later initiation of sexual activity and increased contraceptive use, rather than an increase in abortion rates, mandatory sex education programs, or an increase in abstinence programs. Research and public surveys indicate that comprehensive sex education, which includes information about safer sexual practices and contraception, is favored by the majority of U.S. adults over abstinence-only education, and is deemed more effective in reducing unintended pregnancies and sexually transmitted infections (STIs).