Coupon Collection Behavior: Parents Continue to Reproduce Until They Have Children of Both Sexes
Thu, April 22, 2021

Coupon Collection Behavior: Parents Continue to Reproduce Until They Have Children of Both Sexes



For many centuries, many people wanted their firstborn to be a boy because of a belief that they can ensure the longevity of the family home and they would make better children. Even when they marry and have offspring of their own, their family name still lives on. This gender preference can still be observed in this current time but it did not rule out having a girl altogether.

A new study from the University of Michigan even revealed that some parents continue to reproduce until they have kids of both sexes. The authors referred to it as the “coupon-collection behavior” in human reproduction.


Coupon-collection reproductive behavior

Authors Erping Long and Jianzhi Zhang analyzed sibling records of more than 300,000 people from the UK Biobank database, which contain family, health, and genetic information form volunteer participants. A vast majority of the participants were born between 1940 and 1970. The authors found that coupon-collection behavior in human reproduction increases during the 3 decades. It’s not uncommon to hear that all four or five children of a family are of the same sex and it poses a question whether these families have a low or high probability that a birth yields to a boy instead of a girl. This is why parents continue to reproduce until they have children of both sexes.

Like a coupon collector’s problem, having a complete set of distinct coupons is considered a win. For instance, one continues to buy a particular brand of cereal that contains a random coupon until a person has all different types of coupons in the set.

Zhang, who is a professor in the U-M Department of Ecology and Evolutionary Biology, told Science Daily that such behavior becomes popular only when sons and daughters are believed to have the same utility to families. Based on their findings, there needs to be a society-wide appreciation of gender diversity and improvement in gender equality.

While their study shows a conservative estimate of couples from the United Kingdom, the authors believe that the true fraction of families that are following the coupon-collection reproductive behavior is likely to be higher. Their result is likewise consistent with the interviews conducted in Europe, wherein parents express their desires to have kids of both sexes.

Observations in Finland, Sweden, and Denmark also show the probability of a mother to bear a third child when her first two kids are of the same sex is higher than when they are of different sexes.




Analyzing the tree-based genealogical database in the Netherlands

To confirm their findings, Zhang and Long also analyzed a family tree-based genealogical database, containing 241,000 Dutch families over four centuries. These families have known the number and sexes of children in every family. They first focused on the subset of 12,547 families, in which the youngest child was born between 1940 and 1970 to match the birth year range in their UK Biobank research. A higher-than-expected sex ratio (SR) variation was found over much of the history. It was only after 1940 with the SR variation declined below the expected levels. Thus, the analysis of the Dutch data confirms their finding from the UK Biobank, suggesting that the same reproductive behavior is a relatively recent phenomenon, the authors added.


Can you choose your baby’s gender?

Parents Magazine featured the story of Ellen Durston, who always wanted her first child to be female. She contends that firstborn girls are more confident and ambitious than girls with an older sibling. So, when she and her husband, decided to conceive, she came across in her research a technique pioneered 30 years ago by obstetrics researcher Landrum Shettles, M.D., Ph.D. After following the Shettles method, she became pregnant with a girl.

The Shettles method is known to be a procedure for couples to use before and during intercourse to increase their chances of conceiving a fetus of their desired sex. New York City-based Mount Sinai Beth Israel's obstetrician-gynecologist Masood Khatamee, M.D opined that although Dr. Shettle's method seems to make sense, it still doesn't hold up when it was put to the test.

Meanwhile, the University of Exeter's lecturer in Medical Science Therese Murphy said that the baby’s genetic sex is determined by the man’s sperm as it is the sperm cell that will contain either an X or a Y chromosome. It is the Y chromosome that is essential for the development of the male reproductive organs. So, when it is not present, an embryo will develop into a female.




Global gender ratio

The gender ratio is the ratio between the number of males and females in society. According to scientific online publication Our World in Data, such ratio is not stable but shaped by economic, cultural, social, biological, and technological forces. Most countries have a female share of the population between 49 and 51% but across several countries in East and South Asia, most notably China and India, there are significantly fewer females than males.

Latvia is one of the countries with the highest (54.07%) share of the female population in 2017. Other countries on the list were Russia (53.53%), Portugal (52.67%), and El Salvador (53.04%). In South Korea, the number of male births per 100 female births in the same period was 106.50 (in sex ratio) for the first child, 106.10 for a second child, 105.90 for a third child, and 109.40 for the fourth child and higher. Usually, birth ratios are male-biased, with an expected ratio of 105 male births per 100 female births.


Sex ratios at birth

Pew Research Center also mentioned that the odds that a parent will give birth to a boy or girl depends on which country they live in. However, such differences in the shares of gender relate to the practice of sex-selective abortion that have been identified in the Caucasus and Asia. Government regulations to limit family size due to global economic and social changes seems to likewise contribute to sex-selective abortion and the death of baby girls.

From a genetics point of view, the probability of having a boy or a girl remains the same every time a baby is conceived. It is 50/50, Murphy believes. She is not involved in the University of Michigan study.

Regardless of what gender parents want for their child, it’s more important for babies to be born healthy.