Fertility rates vary both inter- and intra-culturally. Fertility for both males and females is dependent not just on biology but on cultural, religious, economic, and other sociological factors as well. Natural fertility is emphasized in the study of human reproductive ecology. Natural fertility is the measure of human fertility in populations without birth control. Research on natural fertility populations seeks to understand the evolutionary context, ecological constraints, and predict outcomes for human fertility. Fertility is influenced by fecundity, but has additional factors that can increase or decrease an individual's lifetime reproductive success. The inter-birth interval, the amount of time between a woman's births, impacts a woman's total fertility. This amount of time varies cross-culturally, as well as varies with different environmental constraints. Many cultures practice conscious birth spacing to adhere to the desired length of time between pregnancies, or desired number of children. Environmental concerns like fetal loss, lack of resource access, and disease may all impact fertility for females or males. Fertility rates across the globe have steadily declined. This trend, known as the demographic transition, began in the 1700s and continues today. It is strongly correlated with increased industrialization in a society. This trend is now seen in almost all cultures, resulting in some societies with below replacement fertility. Below replacement fertility is when the rate of childbirth in a society is less than the amount needed for each woman to have at least one daughter. Since the chance of having a daughter is 50/50, there must be at least two children for every adult woman in the population. Natural fertility populations In 1961, French demographer Louis Henry introduced the term “natural fertility”. Natural fertility is defined as uncontrolled fertility when the couples do not control the number of children and the family size. Controlled fertility populations use controlled methods to stop having children after reaching a certain number of children. In natural fertility populations, the parity related controls of fertility are not influenced by modern birth controls. Therefore, studying and understanding the age-related changes in fecundity is easier in natural fertility populations in compare to controlled fertility populations. Natural fertility populations deliver an easier platform to study the reproductive behavior which may affect the levels of fertility such as pregnancy loss, time for conception, and length of breastfeeding. In Pennsylvania and Ohio states in the United States, the Amish settlements have been studied to understand the age