Which factor helps determine the fertility rate of a group of people?

Some of the social factors that can influence fertility rates are: race, level of education, religion, use of contraceptive methods, abortion, impact of immigrantion, children as a source of labor (on family farms), children as support for couples at older ages, costs of raising children, female labor force ...

you know they are heterozygous for a trait because they have it as jj or jj.