Birth order studies are a dime a dozen, but scientists have spilled far less ink on sibling configurations. Configurations introduce two new variables, sex and time, to the equation, which complicates things but also creates the possibility of finding more interesting patterns in childhood outcomes and leading to insights that can inform family planning. How should parents space their children to maximize each child’s success? How many children should you have? What sex should you hope for and in what order?
As with all things data-based, the answer here is, of course, it depends. Differently families live in different places and have different amounts of money. That said, there are some consistencies within data sets that are broadly indicative of trends. The particular data set to pay attention to here is a 1972 study of educational outcomes conducted by the Educational Testing Service. Based on test results of students from 82 different types of families, researchers concluded that the ideal student is a boy with two younger siblings. His next sibling must be a boy, fewer than two years younger than he is (to illustrate just how cruel fate can be, if the second born is a girl, her older brother’s academic rank drops to #3 and, if she’s born more than three years after her older sibling, it plummets to #9). The third child must be a girl, born no fewer than three years after her older brother.
This sibling configuration is known as XB-S. The symbol “X” represents the child in question (our firstborn). “B”, without a dash, is a brother born within two years of his older sibling. “-S”, with a dash, is a sister born more than three years after that. (The lowest ranking is BBBBX—a youngest child with four older brothers, all born in quick succession). In our chart below, X is indicated by a grey icon, B is indicated by a male icon, and -S is indicated by a clock followed by a female icon.
The problem, of course, is that the XB-S configuration might mean that your firstborn boy is set for life, but it also might mean that your latter two children (who you, presumably, love too) are screwed. The second-born boy in the XB-S configuration (himself a BX-S, remember?) ranked #16 out of 82 on test scores. And the youngest daughter (a BB-X) ranked #30.
Still, a closer look at the data suggests that XB-S is still the strongest three-sibling configuration, even from the perspective of the seemingly hapless second- and third-born children. Because the highest rank of any second-born child is #15 (that goes to boys with an older brother that is no more than two years their senior and a younger brother no fewer than three years their junior) and the highest ranked third-born child is #29 (also a boy, but with two significantly older sisters), the XB-S setup is about as efficient as is possible. There are, it turns out, many worse ways to go.
If this is starting to sound to you like an unpleasant mixture of science, mathematics, astrology, and betting on horse races, you’re not alone. Debunking birth order studies has become standard research fare and debunking those debunks is a scientific pastime. There is not, in other words, a strong consensus here. In 2016, scientists definitively announced that second children are pretty much the same as everyone else, only to be challenged one year later by an MIT study that found that they were more likely to end up in prison.
At this point it isn’t easy to parse whether birth order matters or, indeed, whether sibling configuration actually has anything to do with academic, financial, or social success. So it’s worth taking each and every birth order study with a grain of salt, and recognizing that there are many other factors that have been proven to determine success—not the least of which is good parenting.
But if you’re desperate for an evidence-based approach to family planning, we might just have one. Have a kid, wait a few months, pop out a boy, take a break for a few years, and then have yourself a girl. “XB-S” seems to work like a charm. At least, according to research conducted 45 years ago.