Fertility factor (demography)

This article is about determinants that are associated with how many children a person will have in total during their lifetime. For determinants of childbearing timing, see Advanced maternal age and Paternal age effect. For the factor in microbiology, see Fertility factor (bacteria).

Fertility factors are determinants of the number of children that a person is likely to have. Fertility factors are mostly positive or negative correlations without certain causations.

Factors generally associated with increased fertility include religiosity,[1] fertile parents,[2][3] intention to have children,[3] marriage[4] and cohabitation,[5] maternal[6] and social[3] support, rural residence,[7] familism,[8] social pressure,[5] homophily,[5] patriarchy,[9] single nuclear family households,[3] and governmental monetary family allowances.[10]

Factors generally associated with decreased fertility include income,[7] education,[7] female labor participation,[11] population control,[12] contraception,[3] partner reluctance to having children,[3] intense[13] or unstable[14] relationships, value changes,[3] higher tax rates,[3] generosity of public pensions,[3] increased female age,[15] and (to a lesser degree) increased male age.[16]

Factors generally associated with increased fertility

Mennonite siblings, Montana 1937.

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".[1]

For many religions, religiosity is directly associated with an increase in the intention to have children.[1] This appears to be the main means by which religion increases fertility.[9] 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.[9] Catholics generally achieve their intended number of children.[9] 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.[9]

It has also been suggested that religions generally encourage lifestyles with fertility factors that, in turn, increase fertility.[17] 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.[18]

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 religious education.[9] Higher income is associated with slightly increased fertility among Catholic couples, but associated with slightly decreased fertility among Protestant couples.[9]

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.[2][3] This correlation is strongest for mother-daughter transmissibility of fertility.[19] The effect is believed to be caused by intergenerational transmission of values, preferences, attitudes, and religiosity,[3] memes that are transmitted from parents to children, having long-term effects analogous to genetics.[3]

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.[3] Birth order does not seem to have any effect on fertility.[9]

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.[19]

Intention to have children

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.[3] In particular, women with a preference for a mommy track rather than becoming career women generally have more children.[3]

A comparison of a survey and registers in Norway found that parents were more likely to realize their fertility intentions than childless respondents.[20] It was suggested that childless individuals may underestimate the effort of having children.[20] On the other hand, parents may better understand their ability to manage another child.[20] Also, parents intending to have children immediately are more likely to achieve this within two years.[20] In contrast, after four years, the fertility rate was found to be higher among those intending to have children in the long term.[20] Stability of fertility intentions further improve the chance to realize them.[21] Such stability of intentions is increased by the belief that having a child will improve life satisfaction and partner relationship.[21]

Chances of realizing fertility intentions are lower in post-Soviet states than in Western European states.[22]

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

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.[4]

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.[25] Another study, on the contrary, found that cohabiting couples in France have equal fertility as married ones.[3] Also, Russians have a higher fertility within cohabitation, while Romanians tend to have a higher proportion of childless marriages than comparable countries.[26]

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.[27] Another study found that Romanian women with little education have about equal fertility in marital and cohabiting partnerships.[28]

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.[6]

Other social support

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.[3]

Rural residence

A family in a rural area of Mozambique, with multiple children.

Total fertility rates are higher among women in rural areas than among women in urban areas, as evidenced from both low-income,[29] middle-income[29] and high-income countries.[3] Studies have suggested that a higher population density is associated with decreased fertility rates.[30] Within urban areas, people in suburbs are consistently found to have higher fertility.[3]

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.[8] 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).[3]

Other factors associated with increased fertility

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

Factors generally associated with decreased fertility

Income

Graph of Total Fertility Rate vs. GDP (PPP) per capita of the corresponding country, 2015.[32] [33]
Main article: Income and fertility

Increased income and human development are generally associated with decreased fertility rates.[7] 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.[3] It has also been theorized that women who can economically sustain themselves have less incentive to become married.[3] 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.[3]

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.[34][35]

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.[4] [7] A substantial proportion of this effect can also be explained by higher levels of unintended pregnancy among women with less education.[4] Female education decreases fertility to a higher extent than male education.[4] [36][37]

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.[4] For men with the same levels of education, the number of children was 1.0 and 1.7, respectively.[4]

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.[3] 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.[37]

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.[9]

Population control

Further information: Human population control

China and India have the oldest and the largest human population control programs in the world.[38] In China, a one-child policy was introduced between 1978 and 1980,[39] and began to be formally phased out in 2015 in favor of a two-child policy.[40] The fertility rate in China fell from 2.8 births per woman in 1979 to 1.5 in 2010.[12] 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.[12] It has thereby been suggested that a decline in fertility rate would have continued even without the strict antinatalist policy.[41]

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.[42][43] Still, India's family planning program has been regarded as only partially successful in controlling fertility rates.[44]

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.[11] 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.[45]

However, for countries in the OECD area, increased female labor participation has been associated with increased fertility.[46]

Causality analyses indicate that fertility rate influences female labor participation, not the other way around.[3]

Women who work in nurturing professions such as teaching and health generally have children at an earlier age.[3] 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.[3]

Age

Cumulative percentage and average age for women reaching subfertility, sterility, irregular menstruation and menopause.[15]

Regarding age and female fertility, fertility starts at onset of menses, typically around age 12-13[47][48][49] Most women become subfertile during the early 30s, and during the early 40s most women become sterile.[15]

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.[16] 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%.[16]

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.[50]

Contraception

Periods of decreased use of contraceptive pills due to fears of side effects have been linked with increased fertility in the United Kingdom.[3] Introductions of laws that increase access to contraceptives have been associated with decreased fertility in the United States.[3] 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.[14] A study in India found an absolute decrease in total fertility rate of 0.34 with the use of contraception.[7] A study in Bangladesh found no significant correlation between contraceptive use and total fertility.[51]

Other biological factors

Further information: Infertility

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.[52] In 10–20% of cases, no cause is found.[52]

The most common cause of female infertility is ovulatory problems, which generally manifest themselves by sparse or absent menstrual periods.[53] Male infertility is most commonly caused by deficiencies in the semen: semen quality is used as a surrogate measure of male fecundity.[54]

Other factors associated with decreased fertility

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

Factors of no or uncertain effect

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,[55] 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.[4]

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:[7]

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.

See also

References

  1. 1 2 3 Hayford, S. R.; Morgan, S. P. (2008). "Religiosity and Fertility in the United States: The Role of Fertility Intentions". Social Forces (primary research article) 86 (3): 1163–1188. doi:10.1353/sof.0.0000. ISSN 0037-7732.
  2. 1 2 Murphy, Michael (2013). "Cross-National Patterns of Intergenerational Continuities in Childbearing in Developed Countries". Biodemography and Social Biology (primary research report) 59 (2): 101–126. doi:10.1080/19485565.2013.833779. ISSN 1948-5565.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Balbo, Nicoletta; Billari, Francesco C. & Mills, Melinda (2013). "Fertility in Advanced Societies: A Review of Research" (review). European Journal of Population 29 (1): 1–38.
  4. 1 2 3 4 5 6 7 8 Martinez, Gladys; Daniels, Kimberly & Chandra, Anjani (2012). "Fertility of Men and Women Aged 15–44 Years in the United States: National Survey of Family Growth, 2006–2010" (primary research report). Division of Vital Statistics, National Center for Health Statistics.
  5. 1 2 3 4 5 6 Fent,Thomas; Diaz, Belinda Aparicio & Prskawetz, Alexia (2013). "Family policies in the context of low fertility and social structure" (primary research article). Demographic Research 29 (37).
  6. 1 2 Schaffnit, S. B.; Sear, R. (2014). "Wealth modifies relationships between kin and women's fertility in high-income countries". Behavioral Ecology (primary research article) 25 (4): 834–842. doi:10.1093/beheco/aru059. ISSN 1045-2249.
  7. 1 2 3 4 5 6 7 Rai, Piyush Kant; Pareek, Sarla & Joshi, Hemlata (2013). "Regression Analysis of Collinear Data using r-k Class Estimator: Socio-Economic and Demographic Factors Affecting the Total Fertility Rate (TFR) in India" (primary research article). Journal of Data Science 11: 323–342. Retrieved 12 April 2016.
  8. 1 2 Hartnett, Caroline Sten; Parrado, Emilio A. (2012). "Hispanic Familism Reconsidered". The Sociological Quarterly (primary research article) 53 (4): 636–653. doi:10.1111/j.1533-8525.2012.01252.x. ISSN 0038-0253.
  9. 1 2 3 4 5 6 7 8 9 10 11 12 Westoff, Charles F.; & Potter, R. G. (2015). Third Child: A Study in the Prediction of Fertility. Princeton, NJ: Princeton University Press. ISBN 9781400876426. Pages 238-244
  10. 1 2 3 Gauthier, A., & Hatzius, J. (1997). "Family benefits and fertility: An econometric analysis" (primary research article). Population Studies 51 (3).
  11. 1 2 Bloom, David E.; Canning, David; Fink, Günther & Finlay, Jocelyn E. (2007). "Fertility, Female Labor Force Participation, and the Demographic Dividend [NBER Working Paper No. 13583]" (primary research, working draft). Cambridge, MA, USA: National Bureau of Economic Research.
  12. 1 2 3 Feng, Wang; Yong, Cai; Gu, Baochang (2012). "Population, Policy, and Politics: How Will History Judge China's One-Child Policy?" (PDF). Population and Development Review 38: 115–29. doi:10.1111/j.1728-4457.2013.00555.x.
  13. 1 2 Rijken, Arieke J.; Liefbroer, Aart C. (2008). "The Influence of Partner Relationship Quality on Fertility [L’influence de la qualité de la relation avec le partenaire sur la fécondité]". European Journal of Population / Revue européenne de Démographie (primary research report) 25 (1): 27–44. doi:10.1007/s10680-008-9156-8. ISSN 0168-6577.
  14. 1 2 3 4 5 6 The ESHRE Capri Workshop Group (2010). "Europe the continent with the lowest fertility" (review). Human Reproduction Update 16 (6): 590–602. doi:10.1093/humupd/dmq023. PMID 20603286.
  15. 1 2 3 te Velde, E. R. (2002). "The variability of female reproductive ageing". Human Reproduction Update 8 (2): 141–154. doi:10.1093/humupd/8.2.141. ISSN 1355-4786.
  16. 1 2 3 Kidd SA, Eskenazi B, Wyrobek AJ (2001). "Effects of male age on semen quality and fertility: a review of the literature". Fertil Steril 75 (2): 237–48. doi:10.1016/S0015-0282(00)01679-4. PMID 11172821.
  17. ↑ Page 118 in: Li Zhang (2010). Male Fertility Patterns and Determinants. Volume 27 of The Springer Series on Demographic Methods and Population Analysis. Springer Science & Business Media. ISBN 9789048189397.
  18. ↑ Joseph G. Schenker1 and Vicki Rabenou (1993). "Family planning: cultural and religious perspectives". Hum. Reprod 8 (6): 969–976. PMID 8345093.
  19. 1 2 Osiewalska, Beata (2013). "Transmission of fertility pattern in mother-daughter relation – Bayesian view (a case study of Austria)" (primary research article). Studia Demograficzne. Retrieved 12 April 2016.
  20. 1 2 3 4 5 Lars Dommermuth, Jane Klobas, Trude Lappegård (2014). "Differences in childbearing by time frame of fertility intention. A study using survey and register data from Norway". Part of the research project Family Dynamics, Fertility Choices and Family Policy (FAMDYN)
  21. 1 2 Laura Cavalli, Jane Klobas (2013). "How expected life and partner satisfaction affect women's fertility outcomes: the role of uncertainty in intentions". Population Review 52 (2).
  22. ↑ Zsolt Spéder, Balázs Kapitány (2014). "Failure to Realize Fertility Intentions: A Key Aspect of the Post-communist Fertility Transition". Population Research and Policy Review.
  23. ↑ Axinn, William G.; Clarkberg, Marin E.; Thornton, Arland (1994). "Family Influences on Family Size Preferences". Demography 31 (1): 65. doi:10.2307/2061908. ISSN 0070-3370.
  24. ↑ Vignoli, Daniele and Rinesi, Francesca and Mussino, Eleonora (2013). "A home to plan the first child? Fertility intentions and housing conditions in Italy". Population, Space and Place.
  25. ↑ Brienna Perelli-Harris (2014). "How Similar are Cohabiting and Married Parents? Second Conception Risks by Union Type in the United States and Across Europe". European Journal of Population. 30=issue 4: 437–464.
  26. ↑ Potarca and Mills and Lesnard (2013). "Family Formation Trajectories in Romania, the Russian Federation and France: Towards the Second Demographic Transition?". European Journal of Population.
  27. ↑ Jan M. Hoem, Cornelia Mureşan, Mihaela Hărăguş (2013). "Recent Features of Cohabitational and Marital Fertility in Romania" (PDF). Population English Edition.
  28. ↑ Hoem, Jan M. and Muresan, Cornelia (2011). "The Role of Consensual Unions in Romanian Total Fertility". Stockholm research reports in demography.
  29. 1 2 Global Monitoring Report 2015/2016: Development Goals in an Era of Demographic Change. World Bank Publications. 2015. ISBN 9781464806704. Page 148
  30. ↑ Sato, Yasuhiro (30 July 2006), "Economic geography, fertility and migration" (PDF), Journal of Urban Economics, retrieved 31 March 2008
  31. ↑ Gandotra, M.M. & Pandey, D. (1982). "Differences in fertility and family planning practices by type of family" (primary research article). Journal of Family Welfare 29 (1): 29–40.
  32. ↑ "Field Listing: Total Fertility Rate". The World Factbook. Retrieved 2016-04-24.
  33. ↑ "Country Comparison: GDP - Per Capita (PPP)". The World Factbook. Retrieved 2016-04-24.
  34. ↑ "The best of all possible worlds? A link between wealth and breeding". The Economist. August 6, 2009.
  35. ↑ Mikko Myrskylä; Hans-Peter Kohler; Francesco C. Billari (6 August 2009). "Advances in development reverse fertility declines". Nature 460: 741–743. doi:10.1038/nature0823.
  36. ↑ Breierova, Lucia Breierova & Duflo, Esther (2004). "The Impact of Education on Fertility and Child Mortality: Do Fathers Really Matter Less Than Mothers? [NBER Working Paper No. 10513]" (primary research, working draft). Cambridge, MA, USA: National Bureau of Economic Research.
  37. 1 2 Kravdal, O.; Rindfuss, R. R. (2008). "Changing Relationships between Education and Fertility: A Study of Women and Men Born 1940 to 1964". American Sociological Review 73 (5): 854–873. doi:10.1177/000312240807300508. ISSN 0003-1224.
  38. ↑ Aqueil Ahmad. "Global Population and Demographic Trends". New Age Globalization (Palgrave Macmillan US): 33–60. doi:10.1057/9781137319494_3. ISBN 978-1-137-31949-4
  39. ↑ Zhu, W X (1 June 2003). "The One Child Family Policy". Archives of Disease in Childhood 88 (6): 463–64. doi:10.1136/adc.88.6.463. PMC 1763112. PMID 12765905.
  40. ↑ Hesketh, Therese; Zhou, Xudong; Wang, Yun (2015). "The End of the One-Child Policy". JAMA 314 (24): 2619. doi:10.1001/jama.2015.16279. ISSN 0098-7484.
  41. ↑ Geoffrey Gilbert (2005). World Population: A Reference Handbook (Contemporary world issues). ABC-CLIO. ISBN 9781851099276. Page 25
  42. ↑ G.N. Ramu (2006), Brothers and sisters in India: a study of urban adult siblings, University of Toronto Press, ISBN 0-8020-9077-X
  43. ↑ Arjun Adlakha (April 1997), Population Trends: India (PDF), U.S. Department of Commerce, Economics and Statistics Administration, Bureau of the Census, retrieved 2009-12-05
  44. ↑ Murthy, N.R.Narayana (2010). Better India, A Better World. Penguin Books India. p. 98. ISBN 9780143068570.
  45. ↑ Lin Lean Lim. "Female Labour-Force Participation" (PDF). United Nations., part of Completing the Fertility Transition. United Nations Publications. 2009. ISBN 9789211513707.
  46. ↑ Namkee Ahn and Pedro Mira (2002). "A Note on the Changing Relationship between Fertility and Female Employment Rates in Developed Countries". Journal of Population Economics 15 (4): 667–682.
  47. ↑ Anderson SE, Dallal GE, Must A (April 2003). "Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart". Pediatrics 111 (4 Pt 1): 844–50. doi:10.1542/peds.111.4.844. PMID 12671122.
  48. ↑ Al-Sahab B1, Ardern CI, Hamadeh MJ, Tamim H. (2010). "Age at menarche in Canada: results from the National Longitudinal Survey of Children & Youth". BMC Public Health 28 (10): 736. doi:10.1186/1471-2458-10-736.
  49. ↑ Hamilton-Fairley, Diana (2004). "Obstetrics and Gynaecology: Lecture Notes" (PDF) (2nd ed.). Blackwell Publishing. Retrieved 2012-08-26.
  50. ↑ K.C. Das, Vandana Gautam, Kumudini Das and P.K. Tripathy (2011). "Influence of age gap between couples on contraception and fertility" (PDF). Journal of Family Welfare 57 (2).
  51. ↑ Wardatul Akmam (2002). "Women's Education and Fertility Rates in Developing Countries, With Special Reference to Bangladesh". Eubios Journal of Asian and International Bioethics 12: 138–143.
  52. 1 2 "ART fact sheet (July 2014)". European Society of Human Reproduction and Embryology.
  53. ↑ "Causes of infertility". National Health Service. Retrieved 15 July 2014.
  54. ↑ Cooper TG, Noonan E, Von Eckardstein S, Auger J, Baker HWG, Behre HM, Haugen TB, Kruger T, Wang C (2009). "World Health Organization reference values for human semen characteristics". Human Reproduction Update 16 (3): 231–45. doi:10.1093/humupd/dmp048. PMID 19934213.
  55. ↑ Geary, David M. (2004). The Origin of the Mind: Evolution of Brain, Cognition, and General Intelligence. American Psychological Association (APA). ISBN 1591471818.
  56. 1 2 Beaujouan, E. (2011). "Second-Union Fertility in France: Partners' Age and Other Factors". Population English edition.
  57. ↑ Krzyżanowska, Monika; Mascie-Taylor, C.G. Nicholas; Thalabard, Jean-Christophe (2015). "Is human mating for height associated with fertility? Results from a british national cohort study". American Journal of Human Biology 27 (4): 553–563. doi:10.1002/ajhb.22684. ISSN 1042-0533.

Further reading

This article is issued from Wikipedia - version of the Tuesday, April 26, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.