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March & Lave's Sons & Daughters Model

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view/download model file: boysngirls.nlogo


In Chapter Three, Evaluation of Speculation, of March & Lave (1975), in section 3.3.3, they examine the consequences of a parental preference for sons:

"Suppose that each couple agreed (knowing the relative value of things) to produce children (in the usual way) until each couple had more boys (the ones with penises) than girls (the ones without).
And further suppose that the probability of such coupling (technical term) resulting in a boy (the ones with) varies from couple to couple, but not from coupling to coupling for any one couple.
And (we still have a couple more) that no one divorces (an Irish folk tale) or sleeps around (a Scottish folk tale) without precautions (a Swedish folk tale).
And that the expected sex (technical term) of a birth if all couples are producing equally is half male, half female (though mostly they are one or the other).

Question: (Are you ready?) What will be the ratio of boys (with) to girls (without) in such a society?

Answer: The sweet truth is (given the supposings) that we will end up with more girls (without) than boys (with).

(That's beauty, baby.):" (pp.69-70)


Each couple is modelled as a turtle. Each couple has a fixed, small bias (mean 0.0, max +0.1, min -0.1) in its probability (unbiassed mean = 0.5) of producing sons over daughters. Each couple remembers the numbers of offspring of each sex it has. Each couple is unhappy until it has more sons than daughters.


Choose the number of couples. Choose the fixed bias per couple (vary_bias of 0 means a probability of exactly 0.5, max 10 means a probability in the range 0.4 to 0.6).

The output box and plots are self-explanatory.


With zero bias, there will almost always be more boys than girls in total, when all couples are happy. With large bias, the process will not converge. Instead, there will remain a very small number of couples coupling, and girls will increasingly outnumber boys. The couples more likely to have sons will have smaller families, and those more likely to produce daughters will have larger families. Some with many daughters and few sons remain unhappy and coupling.


Play with numbers of couples and vary_bias.


Although couples are modelled as turtles, there is no movement of turtles. Instead, couples could be modelled as patches, and the colour of a patch could change when a couple becomes happy.


Note the use of the false attribute in the initiation setup.


This section could give the names of models in the NetLogo Models Library or elsewhere which are of related interest.


Model written by Robert Marks, on August 10, 2006.

James G. March and Charles A. Lave, Introduction to Models in the Social Sciences, New York: HarperCollins, 1975