The parents of large families have sons and daughters in every combination and birth order. But some couples seem to go on a gender-streak. After Joe and Rose Kennedy had two sons, Joe Jr. and the future president, John F. Kennedy, they went on a streak of four girls in a row before alternating boy-girl-boy for the three youngest. Joe and Katherine Jackson, the parents of the Jackson 5, started with four boys, then had their first girl, LaToya, and then had another four boys before Janet.
What are the chances of that? Not all that low.
The odds of having one child of either gender are nearly, but not exactly, 1 in 2. US birth statistics reveal the odds are slightly in favor of a baby’s being male: roughly 105 boys are born for every 100 girls. So the odds a newborn is male are 1 in 1.95 (51%), while the odds a newborn is female are slightly lower, 1 in 2.05. This means that, when it comes to a woman’s first n children, streaks of daughters are slightly rarer than those of boys.
Having already had several children of one sex does not affect the gender of the next child. The reason is that meiosis makes every baby’s odds of being a particular gender roughly 1 in 2. As they become spermatozoa or eggs, male and female reproductive cells divide their 46 chromosomes into two cells with 23 chromosomes. In women, who have two X chromosomes, their reproductive cells (oocytes) all contain one X chromosome. Men, who have one X and one Y chromosome, produce equal numbers of X-laden and Y-laden sperm: an XY cell doubles itself, becoming an XY and an XY (meiosis I), then each XY undergoes a further division (meiosis II), making an X, a Y, an X, and a Y. The only males who produce irregular chromosomal ratios are those with genetic disorders, like Klinefelter syndrome, that result in sperm with an irregular number of sex chromosomes. Men with Klinefelter syndrome are sterile. So neither men nor women have a biological predisposition to conceiving children of one sex.
To calculate the odds a woman’s first n children will be one gender, use these formulae: 1/ 0.5119ˆn for boys, and 1/0.4885ˆn for girls. So the odds a woman’s first 2 children will be boys are 1 in 3.82, and the odds a woman’s first 2 children will be girls are 1 in 4.19. As streaks lengthen, the odds diverge by gender. Thus, the odds a woman’s first
3 children will be boys = 1 in 7.45 3 children will be girls = 1 in 8.58
4 children will be boys = 1 in 14.56 4 children will be girls = 1 in 17.56
5 children will be boys = 1 in 28.45 5 children will be girls = 1 in 35.95
6 children will be boys = 1 in 55.57 6 children will be girls = 1 in 73.6
And so on. Some families with long gender-streaks:
Ray Davies, lead singer of The Kinks, has six consecutive older sisters.
The Duggars of Arkansas, known for their TLC show 18 Kids and Counting: while not having a streak of 18, the Duggars do have 6 boys in a row—Jedidiah, Jeremiah, Jason, James, Justin, and Jackson. The chances of that particular arrangement are one in 8.24, roughly the odds a person lives in California (1 in 8.27).
Not only were Grover C. Jones, Sr.’s first 16 children boys—at odds of one in 44,962—but he was also the finder, along with his son “Punch,” of the largest alluvial diamond ever found in America. While playing horseshoes in 1928, Grover and Punch found a bluish rock, which they believed to be quartz. They placed it in a cigar box, where it stayed for 14 years while the Joneses struggled through the entirety of the Great Depression. In 1942, at Punch’s insistence, Grover brought it to a geology professor at Virginia Polytechnic Institute, who determined that it was in fact a 34.48 carat diamond. The Joneses did not sell the diamond until 1984.
If you’re beginning to suspect that consecutive sons/daughters are not so uncommon, you’re correct. After all, the odds a woman’s first 5 children will be female are only slightly lower than the odds a baby will be part of the birth of twins (1 in 31.12). Multiple births, especially those of higher order (triplets and beyond), are much rarer than streaks.
Consider the ultimate multiple mother, Mrs. Feodor Vassilyev (of late 18th-century Russia), who gave birth to more children than any woman in recorded history— 69 of them. In 27 total “confinements,” she had 16 sets of twins, 7 sets of triplets, and 4 sets of quadruplets. Even by today’s standards of fertility, Mrs. Vassilyev is a marvel of fruitfulness and multiplication. The odds today of having a set of twins are about 3.15%, or roughly 1 in 32. Of triplets: 0.17%, or 1 in 575. Of quadruplets: 0.01%, or 1 in 8,739. So the odds of having a family exactly like Mrs. Vassilyev’s are truly galactic—1 in 2.05 x 10^49, or 1 in 20,500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.