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What Is the Distribution of Offspring Per Individual?
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A commenter below notes:

Also, in modern society, doesn’t just about everyone reproduce, such that not only is any particular advantage competing against other countervailing pressures as you note, but also that the “less fit” genomes are not removed from the overall population, but rather are added back to the mix? In other words, the less-preferred short males don’t die and have zero kids, they also get married and their genes get thrown back into the pot.

First, let’s not get caught in the assumption that for genes to be disfavored one has to have zero fitness in individuals carrying those genes. If, for example, in a situation of demographic expansion you had individuals who had eight children vs. those who had one child, there would be selection for the traits which were passed by those with eight children in relation to those who had one child. But, it did make me realize I wasn’t intuitively aware of the distribution of number of offspring in the population. I assumed that the median was around two, but that’s about it.

So, I looked at the GSS CHILDS variable for individuals born in 1950 or earlier from the year 2000 on (COHORT and YEAR variables). I also separated out the results by sex. Do not take these results as definitive because the GSS data set is not entirely representative. But, it does give you a general sense.

In hindsight I can’t say I’m surprised that somewhat over 10 percent of middle-aged adults and older currently don’t have any children. That sounds about right. And the proportion of those who were only children also seems plausible.

(Republished from Discover/GNXP by permission of author or representative)
• Category: Science • Tags: Data, Data Analysis 
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  1. So when you get back to the original question, does tall mean “fit”, to get really precise, the question is, does tall mean more babies? I think in your original post your answer was, it’s not that simple, correct?

  2. #2, control-f how many times i said reproductive in that post! on this blog when i say ‘fit’ i men reproductive fitness. in any case, the point is that fitness may be conditional on sex and frequency dependence….

  3. I expected a heavier right tail for men than women. Interesting that the distribution is about the same.

  4. #3, I briefly had a similar reaction. However, the sample is of individuals born before 1950, so that might have something to do with it. Or, our intuition may be just wrong.

  5. I find the consistency between the tables amazing. Compare to sex partner count where men consistenly report more than women. I guess children are just easier to count.

  6. * The GSS numbers look like they are far too heavily weighted towards large families (or are sorely lagging indicators picking up trends that have since faded). GSS has about 27% of people having four or more children. The percentage of all births that are fourth or higher ordinal births in 1998 was on the order of 7.1% , which is closer to 6.2% with a more accurate modern figure of 20% childlessness. This is less than a quarter of the GSS number. Query if GSS is capturing people who are counting stepchildren that Census numbers do not include.

    By comparison, while 20% of women never have children, the percentage who never marry is closer to 5% and many never married women have children. A disproportionate share of women who choose to never have children are or have been married.

    * Family size has varied a great deal over the relevant time period with a bust in the Great Depression and War Years, followed by the Baby Boom, and then some secondary bumps up and down over time. Consider this quote from a 2008 article in the New York Times:

    “Twenty percent of women ages 40 to 44 have no children, double the level of 30 years ago. . . and women in that age bracket who do have children have fewer than ever — an average of 1.9 children, compared with the mean average of 3.1 children in 1976.”

    Bumps in total family size are not neutral with respect to the distribution of number of children. Bumps up and down disproportionately affect the right tail.

    * In Japan, the number of child per married woman has remained almost constant in the post-war period, but the number of never married women has soared (and non-marital childbearing is more rare in Japan than almost any other developed country), a trend very different than that of the U.S. that accounts for most of the relative lifetime fertility rate differences between the two countries.

    * Fertility treatments have also started to create a bimodal distribution of number of children, with unassisted reproduction gradually getting a thinner right tail, and assisted reproduction creating a multiple birth heavy bump in the right tail made up of the children of women in their late 30s and early 40s.

    * The two big questions in fertility I’d be interesting in knowing pertain to the lifetime fertility of men that due to inperfect assignment of paternity that is common in some of the most interesting cases is somewhat hard to gather data upon.

    First, I’d be interested to know what the relative liftime fertility of men with adult felony convictions is relative to those who don’t have adult felony convictions.

    Second, I’d be interested to know what the relatively lifetime fertility of men who have children with only one marriage and no non-marital children is relative to men who have multiple marriages and/or both non-marital children.

    Put another way, I’m interested in how “not playing by the rules” in our society relates to population genetic fitness.

    * Starting about ten or twenty years ago, the lifetime fertility of women of higher socioeconomic status started to exceed that of women of lower socioeconomic status, mostly due to fertility treatments compensating for delayed child bearing in higher SES women and reduced fertility among lower SES women. The latter trend also overlaps with the rise of mass incarceration in the United States. For the century before the 1990s, lower SES women have more lifetime children and had them younger than higher SES women.

    * One of the less obvious facts about population genetic fitness is how important the age at which one has children is in addition to the number of children per generation. Even if all women in two groups have precisely two children per lifetime, the difference between having children at age 20 and having children at age 30 is the difference between having two generation and three generations in the same sixty year period, which means that the women with earlier childbearing age have a number of descendants after sixty years (8) similar to the number that would be had by people who had children at age 30 with three children each after sixty years (9). Shorter generation length gives you a selective advantage if everyone in a generation survives to reproduce at the same frequency as their parents in both cases equivalent to another full child per generation.

    The percentage of people who are childless, the distribution of the number of children of people who have children by number of children, and the age of child bearing all factor into population genetic fitness.

  7. Very interesting. Here are the numbers from Norway (table is in English):

    In the 1940 male cohort, 13.9% had zero kids at the age of 45. In the 1961 male cohort, 22.2% had zero kids at the age of 45. The increase between the two cohorts is 60%.

    In the 1940 female cohort, 9.5% had zero kids at the age of 45. In the 1960 female cohort, 11.9% had zero kids at the age of 45. The increase between the two cohorts is 25%.

    There has been a few articles in Norwegian media recently about a new trend where young women search out “ligthly used” men that are already fathers. These women are not neccessary looking for older or richer men, just men that are already fathers and have proven their fathering skills. A increasing number of men therefore have two sets of kids, while an increasing number of men have no kids. I think this may have genetic implications, since the process doesn’t seem to be random to begin with.

  8. ohwilleke: The percentage of all births that are fourth or higher ordinal births in 1998 was on the order of 7.1%… This is less than a quarter of the GSS number.

    Those don’t sound like comparable numbers. If everyone has exactly 4 births, the GSS number will be 100%, but only 25% of births will be 4th births. From the GSS numbers, 100 mothers produce 272 births, of which 55 are 4th or later. 55/272=20%. I don’t see how you get 7.1% out of your link. It looks like 14% to me: (5.7+2.6+1.7+0.6)/71.

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