The Birthday Effect in Lottery Numbers: Why Picks Cluster Between 1 and 31
A friend of mine has bought lottery tickets the same way for ten years: every number comes from a family birthday — his father’s month, his mother’s date, his child’s day of birth, until six are filled. Last week he told me, looking pleased, that if he won it would be “a win for the whole family”.
I did not have the heart to answer straight away. I went and looked at the data first.
Two countries, one hot week for lotteries
- Brazil: in the Mega-Sena 3,057 draw reported by Brazilian media on Sunday 13 September, the six numbers were 14, 21, 25, 40, 51 and 56. Nobody matched all six, so the jackpot rolled over to 95 million reais, with the next draw on 15 September. One player in Santa Catarina missed by a single number; each such ticket paid just 31,872.72 reais.
- Korea: in the 1,241st draw on 12 September, the winning numbers were 7, 13, 16, 23, 24 and 43, with 9 as the bonus number. Eighteen tickets shared first prize, each worth 1.63 billion won, and 13 of them were quick-pick tickets while 5 were written by hand.
Researchers gave this a name
My friend’s habit is not a personal quirk. The Journal of Gambling Studies has published a paper devoted to the “birthday-number effect”, and its opening states the finding plainly: players do not choose at random — the phenomenon is called conscious selection, and the main bias is a preference for small numbers.
It is not hard to guess why. The numbers a person can reel off from memory are almost all governed by a calendar: at most 12 months, at most 31 days. So the numbers from 1 to 31 get picked again and again, while anything from 32 upwards stays quiet. Winning is random, but how many people share the same pot is not: the more popular your numbers, the thinner the payout even if you win. Someone who picks birthdays is quietly adding strangers to the split.
Number preferences were never only about probability
Look one layer further out and the feelings people have about numbers run deep:
- Four in East Asia: in Chinese, Japanese and Korean, the word for “four” sounds close to “death”, so hospitals skip the number and apartment blocks have no fourth floor.
- Thirteen in the West: from the thirteenth guest at the Last Supper to hotels with no thirteenth floor, the taboo is alive in Britain, America and Latin America alike.
- The double life of eight: favoured in Chinese-speaking circles for the way it sounds, and an ordinary digit everywhere else.
None of these preferences can change a draw machine, but they do change people. The machine only does random; when a human picks, the hand reaches for whatever carries a memory.
I sent the data to my friend. He was quiet for a while and then wrote: “So my numbers are numbers a lot of other people have?” Yes. Then he asked a better question: “So how should I pick?” The answer is not to change the numbers, but to decide first whether you are buying probability or buying a small hope. Both are legitimate, but it is best not to mix the arithmetic. If you want to work through something that keeps circling your mind, write the question down on 占卜迷指, take the corresponding chart and reading through the real flow, and export it — that beats running it around your head a few more times.
Disclaimer: This article covers traditional culture and folklore and is intended for entertainment and self-reflection only. It is not medical, psychological, investment, legal or marriage advice. Do not submit ID numbers, detailed addresses or other sensitive personal information. If you experience persistent insomnia, anxiety or physical discomfort, please see a doctor or a qualified professional.
Sources
- D’Hondt, Roger, Hoffmann, Plotkina, “Is There a Gender Gap in the Birthday-Number Effect?”, Journal of Gambling Studies (2024): https://doi.org/10.1007/s10899-024-10288-5
- Mega-Sena 3,057 draw results and rolled-over jackpot (13 September 2026): https://www.estadao.com.br/einvestidor/loterias/resultado-da-mega-sena-3057-confira-as-dezenas-sorteadas-neste-domingo-13/
- g1, “Santa Catarina tickets one number short” (13 September 2026): https://g1.globo.com/sc/santa-catarina/noticia/2026/09/13/mega-sena-apostas-sc-ficam-a-um-numero-de-faturar-r-84-milhoes.ghtml
- 머니투데이, “Winning numbers for the 1,241st Lotto draw: 7·13·16·23·24·43” (12 September 2026): https://www.mt.co.kr/society/2026/09/12/2026091220340429608
- Korea Daily, “18 first-prize winners in draw 1,241, 1.63 billion won each”: https://www.koreadaily.com/article/20260912053054448