Fertility factor (demography)

Intention to have children

The predictive power of intentions continues to be debated. Research that argues that intentions are a good predictor of actual results tends to draw ideas from the Theory of planned behavior (TPB). According to TPB, intentions stem from 1) attitudes regarding children, including the cost of raising them versus perceived benefits, 2) subjective norms, for example, the influence of others, and 3) perceived control over behavior, that is, how much control does an individual have over their own behavior.

Fertility intentions tend to boil down to how many children to bear (called quantum intentions), and when to have them (called tempo intentions). Of these, quantum intention is a poor predictor because it tends to change as a result of the ups and downs of a typical life. Timing intentions are a somewhat better predictor, but still a poor way to predict actual results.

Intention to have children generally increases the probability of having children. This relation is well evidenced in advanced societies where birth control is the default option. In particular, women with a preference for a mommy track rather than becoming career women generally have more children.

A comparison of a survey to birth registers in Norway found that parents were more likely to realize their fertility intentions than childless respondents. It was suggested that childless individuals may underestimate the effort of having children. On the other hand, parents may better understand their ability to manage another child. Also, parents intending to have children immediately are more likely to achieve this within two years. In contrast, after four years, the fertility rate was found to be higher among those intending to have children in the long term. Stability of fertility intentions further improve the chance to realize them. Such stability of intentions is increased by the belief that having a child will improve life satisfaction and partner relationship.

Chances of realizing fertility intentions are lower in post-Soviet states than in Western European states.

There are many determinants of the intention to have children, including:

Religiosity

A survey in the United States in 2002 found that overall, women who reported religion as "very important" in their everyday lives had higher fertility than those reporting it as "somewhat important" or "not important".

For many religions, religiosity is directly associated with an increase in the intention to have children. This appears to be the main means by which religion increases fertility. For example, Catholic couples generally have intentions to have more children than Jewish couples, who in turn generally intend to have more children than Protestant couples. Catholics generally achieve their intended number of children. Among Catholics, increased religiosity is associated with the intention to have more children, while increased religiousness among Protestants is associated with intention to have fewer children.

It has also been suggested that religions generally encourage lifestyles with fertility factors that, in turn, increase fertility. For example, religious views on birth control are, in many religions, more restrictive than secular views, and such religious restrictions have been associated with increased fertility.

Religion sometimes modifies the fertility effects of education and income: Catholic education at the university level and, to a lesser degree, at the secondary school level, is associated with higher fertility, even when accounting for the confounding effect that higher religiosity leads to a higher probability of attending a religiously affiliated school. Higher income is associated with slightly increased fertility among Catholic couples, but associated with slightly decreased fertility among Protestant couples.

Parents’ religiosity is positively associated with their children’s fertility. Therefore, more religious parents will tend to increase fertility.

Fertile parents

In high-income countries, the number of children a person has strongly correlates with the number of children that each of those children will eventually have. This correlation is strongest for mother-daughter transmissibility of fertility. The effect is believed to be caused by intergenerational transmission of values, preferences, attitudes, and religiosity, memes that are transmitted from parents to children, having long-term effects analogous to genetics.

Danish data from non-identical twins growing up in the same environment compared to identical twins indicated that genetic influences in themselves largely override previous shared environmental influences. Birth order does not seem to have any effect on fertility.

The correlation between the fertility of parents and that of the children seems to be insignificant in low-income countries, in particular those that have not undergone a demographic transition.

Marriage and cohabitation

A large survey in the United States found that married women had an average of 1.9 children, compared to 1.3 among those cohabiting. The corresponding numbers for men were 1.7 and 1.1, respectively. The difference of 0.6 children for both sexes was expected to decrease to between 0.2 and 0.3 over a person's lifetime when correcting for the confounder that married people have their children earlier in life.

A study of the United States and multiple countries in Europe found that women who continue to cohabit after giving birth have significantly lower probability of having a second child than married women in all countries except those in Eastern Europe. Another study, on the contrary, found that cohabiting couples in France have equal fertility as married ones. Also, Russians have a higher fertility within cohabitation, while Romanians tend to have a higher proportion of childless marriages than comparable countries.

Survey data from 2003 in Romania came to the result that marriage equalized the total fertility rate among both highly educated and limited-education people to approximately 1.4. Among those cohabiting, on the other hand, a lower level of education increased fertility rate to 1.7, and a higher level of education decreased it to 0.7. Another study found that Romanian women with little education have about equal fertility in marital and cohabiting partnerships.

Maternal support

Data from the Generations and Gender Survey showed that women with living mothers had earlier first births, while a mother's death early in a daughter's life correlated with a higher probability of childlessness. On the other hand, fathers' survival had no effect on any outcome. Co-residence with parents delayed first births and resulted in lower total fertility and higher probability of childlessness; this effect is even stronger for poor women.

Other social support

Social support from the extended family and friends can help a couple decide to have a(other) child. For example,

Studies mainly in ex-communist Eastern European countries have associated increased fertility with increased social capital in the form of personal relationships, goods, information, money, capacity to work, influence, power, and personal help from others.

Research in the U.S. shows that the extended family willing to provide support becomes a “safety net”. This is particularly important for single mothers and situations involving partnership instability.

Rural residence

Total fertility rates are higher among women in rural areas than among women in urban areas, as evidenced from both low-income, middle-income and high-income countries. Studies have suggested that a higher population density is associated with decreased fertility rates. Within urban areas, people in suburbs are consistently found to have higher fertility.

Familism

Limited support has been found among Hispanics of an increased fertility with increased familism, prioritizing the needs of the family over that of individuals. The fertility impact is unknown in country-level familism systems where the majority of the economic and caring responsibilities rest on the family (such as in Southern Europe), as opposed to defamilialized systems where welfare and caring responsibilities are largely supported by the state (such as Nordic countries).

Other factors associated with increased fertility

Income

Increased income and human development are generally associated with decreased fertility rates. It has been theorized that people earning more have a higher opportunity cost if they focus on childbirth and parenting rather than their continued careers. It has also been theorized that women who can economically sustain themselves have less incentive to become married. For women, education modifies the effect of economic uncertainty, wherein mothers with less education usually respond to economic uncertainty by adopting the role of mothers, while highly educated women usually respond to it by postponing childbearing.

On the other hand, there is some evidence that with rising economic development, fertility rates drop at first, but then begin to rise again as the level of social and economic development increases, while still remaining below the replacement rate.

Education

An increased level of education is associated with decreased fertility. It has been suggested that this decrease results from an increase in the ages of marriage and childbearing, and in the likelihood of contraceptive use. A substantial proportion of this effect can also be explained by higher levels of unintended pregnancy among women with less education. Female education decreases fertility to a higher extent than male education.

In the United States, a large survey found that women with a bachelor's degree or higher had an average of 1.1 children, while those with no high school diploma or equivalent had an average of 2.5 children. For men with the same levels of education, the number of children was 1.0 and 1.7, respectively.

In Europe, on the other hand, women who are more educated eventually have about as many children as do the less educated, but that education results in having children at an older age. Likewise, a study in Norway found that better-educated males have a decreased probability of remaining childless, although they generally became fathers at an older age.

Catholic education at the university level and, to a lesser degree, at the secondary school level, is associated with higher fertility, even when accounting for the confounding effect that higher religiosity among Catholics leads to a higher probability of attending a religiously affiliated school.

Population control

China and India have the oldest and the largest human population control programs in the world. In China, a one-child policy was introduced between 1978 and 1980, and began to be formally phased out in 2015 in favor of a two-child policy. The fertility rate in China fell from 2.8 births per woman in 1979 to 1.5 in 2010. However, the efficacy of the one-child policy itself is not clear, since there was already a sharp reduction from more than five births per woman in the early 1970s, before the introduction of the one-child policy. It has thereby been suggested that a decline in fertility rate would have continued even without the strict antinatalist policy.

Extensive efforts have been put into family planning in India. The fertility rate has dropped from 5.7 in 1966 to 2.7 in 2009. Still, India's family planning program has been regarded as only partially successful in controlling fertility rates.

Female labor participation

Increased participation of women in the workforce is associated with decreased fertility. A multi-country panel study found this effect to be strongest among women aged 20–39, with a less strong but persistent effect among older women as well. International United Nations data suggests that women who work because of economic necessity have higher fertility than those who work because they want to do so.

However, for countries in the OECD area, increased female labor participation has been associated with increased fertility.

Causality analyses indicate that fertility rate influences female labor participation, not the other way around.

Women who work in nurturing professions such as teaching and health generally have children at an earlier age. It is theorized that women often self-select themselves into jobs with a favorable work–life balance in order to pursue both motherhood and employment.

Age

Regarding age and female fertility, fertility starts at onset of menses, typically around age 12-13 Most women become subfertile during the early 30s, and during the early 40s most women become sterile.

Regarding age and male fertility, men have decreased pregnancy rates, increased time to pregnancy, and increased infertility as they age, although the correlation is not as substantial as in women. When controlling for the age of the female partner, comparisons between men under 30 and men over 50 found relative decreases in pregnancy rates between 23% and 38%.

An Indian study found that couples where the woman is less than one year younger than the man have a total mean number of children of 3.1, compared to 3.5 when the woman is 7–9 years younger than the man.

Contraception

Periods of decreased use of contraceptive pills due to fears of side effects have been linked with increased fertility in the United Kingdom. Introductions of laws that increase access to contraceptives have been associated with decreased fertility in the United States. However, short-term decreases in fertility may reflect a tempo effect of later childbearing, with individuals using contraceptives catching up later in life. A review of long-term fertility in Europe did not find fertility rates to be directly affected by availability of contraceptives. A study in India found an absolute decrease in total fertility rate of 0.34 with the use of contraception. A study in Bangladesh found no significant correlation between contraceptive use and total fertility.

Other biological factors

20-30% percent of infertility cases are due to male infertility, 20–35% are due to female infertility, and 25-40% are due to combined problems. In 10–20% of cases, no cause is found.

The most common cause of female infertility is ovulatory problems, which generally manifest themselves by sparse or absent menstrual periods. Male infertility is most commonly caused by deficiencies in the semen: semen quality is used as a surrogate measure of male fecundity.

Other factors associated with decreased fertility

Intelligence

The relationship between fertility and intelligence has been investigated in many demographic studies; there is no conclusive evidence of a positive or negative correlation between human intelligence and fertility rate. Survival rates are, however, correlated with IQ, but the net effect on population intelligence is unclear. It is theorized that if an inverse correlation of IQ with fertility rate were stronger than the correlation of survival rate, and if heritable factors involved in IQ were consistently expressed in populations with different fertility rates, and if this continued over a significant number of generations, it could lead to a decrease in population IQ scores. However, the Flynn effect proposes a clear general increase in measured IQ scores over time.

Other correlates of IQ include income and educational attainment, which are also inversely correlated with fertility rate, and are to some degree heritable, and thereby act as confounders.

Other factors of no or uncertain effect

The following have been reported, at least in the primary research literature, to have no or uncertain effects.

Racial and ethnic factors

In the United States, Hispanics, and African Americans have earlier and higher fertility than other racial and ethnic groups.

Multifactorial analyses

A regression analysis on a population in India resulted in the following equation of Total Fertility Rate, where parameters preceded by a plus were associated with increased fertility, and parameters preceded by a minus were associated with decreased fertility:

Total Fertility Rate = 0.02 (human development index*) + 0.07 (infant mortality rate*) − 0.34 (contraceptive use) + 0.03 (male age at marriage*) − 0.21 (female age at marriage) − 0.16 (birth interval) − 0.26 (use of improved water quality) + 0.03 (male literacy rate*) − 0.01 (female literacy rate*) − 0.30 (maternal care)

* = Parameter did not reach statistical significance on its own

Effects

Changes in fertility factors have a major role in demographic transitions, typically from high birth and death rates to low birth and death rates as countries develop from pre-industrial to industrialized economic systems.

Lower fertility can lead to sub-replacement fertility, a total fertility rate that, if sustained, leads to each new generation being less populous than the previous one.