It seems that you have missed the given options for this question which are the following: a) the increase in religious-based sex-abstinence programs b) the availability and use of effective contraceptives c) the decrease in the number of people getting married <span> d) an increase in same-sex couplings Based on this, the reason that is most likely why the dramatic decrease in the number of children in the average American family is the </span>the availability and use of effective contraceptives.