Which U.S. birthdays became more common, and which fell back?
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- January 11largest rise in share+11.4%, 2→1 weekend days
- January 2largest fall in share−12.1%, 1→2 weekend days
- 17.0 → 18.2spread between datesstandard deviation of shares, 365 every-year dates
- 2000–2004 vs 2010–2014two windows, one sourceSSA daily file, one continuous series
Inside one administrative series, the SSA daily file for 2000–2014, each date’s share of its own year’s births can be averaged over 2000–2004 and over 2010–2014 and compared. On that measure January 11 rose the most (+11.4%) and January 2 fell the most (−12.1%). Every table below also counts how many Saturdays and Sundays the date fell on in each window, because that alone moves a date’s count.
How the comparison is built
A share removes the change in annual birth totals: it is the date’s births divided by that year’s births, in parts per 100,000, then averaged across the five years of each window. The two windows are ten years apart inside the same file, with one source, coverage and definition throughout. February 29 is shown separately: it occurred twice in the first window and once in the second, so its figure rests on three days.
Weekend composition is the first thing to read beside any move. In this series a Saturday or Sunday records roughly 63% of a weekday’s births, so a date that fell on three weekend days in one window and one in the other will move by several per cent from the calendar alone. The columns give the counts.
Dates whose share rose
Ten largest rises: share per 100,000, 2000–2004 to 2010–2014
One line per date from its early-window share to its late-window share.
| Date | 2000–2004 | 2010–2014 | Change | Weekend days 00–04 | Weekend days 10–14 | |
|---|---|---|---|---|---|---|
| January 11 | 256.8 | 286.1 | +11.4% | 2 of 5 | 1 of 5 | |
| November 11 | 263.3 | 292.2 | +11.0% | 2 of 5 | 1 of 5 | |
| September 2 | 250.8 | 276.2 | +10.1% | 2 of 5 | 1 of 5 | |
| February 17 | 251.3 | 275.6 | +9.7% | 2 of 5 | 1 of 5 | |
| December 2 | 263.8 | 287.4 | +9.0% | 2 of 5 | 1 of 5 | |
| February 8 | 259.8 | 282.9 | +8.9% | 2 of 5 | 1 of 5 | |
| February 1 | 257.0 | 279.7 | +8.8% | 2 of 5 | 1 of 5 | |
| November 23 | 231.3 | 251.3 | +8.7% | 2 of 5 | 2 of 5 | |
| January 4 | 262.4 | 285.1 | +8.7% | 2 of 5 | 1 of 5 | |
| September 16 | 283.4 | 307.4 | +8.5% | 2 of 5 | 1 of 5 |
November 11 is in this list; the late window contains November 11, 2011, and the series records the day’s count.
Dates whose share fell
Ten largest falls: share per 100,000, 2000–2004 to 2010–2014
One line per date from its early-window share to its late-window share.
| Date | 2000–2004 | 2010–2014 | Change | Weekend days 00–04 | Weekend days 10–14 | |
|---|---|---|---|---|---|---|
| January 2 | 246.7 | 216.7 | −12.1% | 1 of 5 | 2 of 5 | |
| February 20 | 282.0 | 250.5 | −11.2% | 1 of 5 | 2 of 5 | |
| February 27 | 277.9 | 248.9 | −10.4% | 1 of 5 | 2 of 5 | |
| February 13 | 272.2 | 243.9 | −10.4% | 1 of 5 | 2 of 5 | |
| September 11 | 295.2 | 264.5 | −10.4% | 1 of 5 | 2 of 5 | |
| February 6 | 276.9 | 248.2 | −10.4% | 1 of 5 | 2 of 5 | |
| January 30 | 275.6 | 247.5 | −10.2% | 1 of 5 | 2 of 5 | |
| January 16 | 273.1 | 246.0 | −9.9% | 1 of 5 | 2 of 5 | |
| May 8 | 282.6 | 254.8 | −9.8% | 1 of 5 | 2 of 5 | |
| April 17 | 281.6 | 254.1 | −9.8% | 1 of 5 | 2 of 5 |
February 29, on its own
Leap day occurred in 2 of the 2000–2004 years and 1 of the 2010–2014 years. Its share was 230.5 per 100,000 in the first window and 276.0 in the second (+19.7%), with 1 and 0 weekend occurrences. Three days is too few to rank beside dates observed five times in each window, which is why it is kept out of the lists above.
Did the calendar get flatter?
The standard deviation of the per-date share across the 365 dates that occur every year is 17.03 in 2000–2004 and 18.20 in 2010–2014: the spread between dates is slightly wider in the later window. The same question for England & Wales, over two full decades, is on the frequency-change page.
Check how rare your own birthday is → · Births by weekday, same file
Sources, method and limits
Source: Social Security Administration daily births for the United States, 2000–2014, as compiled by FiveThirtyEight (CC BY 4.0). Both windows are complete years inside that one file. Weekday is taken from each record’s calendar date.
Every figure above comes from the administrative records named here, at the granularity those records publish: a day, a month or a year. Coverage and definitions differ between countries and eras, so two sources are combined only where their documentation states the windows are separate, and the page where that happens says so.
Full method, leap-day treatment and definitions.
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