Last week, I was presenting at a conference and discussing the merits of animated visualizations vs. small multiples. On one of my slides, I presented the following chart that shows the total fertility rate (i.e., the average number of children born per woman) for the U.S.A. and Japan over a 60-year time period.

After the talk, one of the audience members came up to me and asked why there was that weird blip in Japan’s fertility rate in 1966. It turns out that there’s a fascinating explanation — an explanation that finds its roots in astrology and superstition.

Astrology and superstition

If you were born in the U.S. (or many other Western countries), you were probably assigned an astrological sign based on the day you were born: Aries if you were born between March 20 and April 19 (roughly), Taurus if you were born between April 19 and May 20, and so on. Each of these signs are associated with personality traits and various other features.

The Japanese use a similar astrological system, but one instead based on the Chinese zodiac. Along with assigning an astrological beast based on your birth year, each year also has one of the Five Elements associated with it—all that dramatically affect what your astrological sign entails.

Astrologers would like us to believe that our personality—and even our entire lives—are guided by these signs, but most people don’t take these predictions too seriously. In 1966, however, many Japanese families were still quite superstitious—and that’s why we see that blip in fertility rates in 1966.

You see, 1966 was the year of 丙午 (Hinoe-Uma), or the “Fire Horse.” As one source describes:

Girls born in [1966] became known as ‘Fire Horse Women’ and are reputed to be dangerous, headstrong and generally bad luck for any husband. In 1966, a baby’s sex couldn’t be reliably detected before birth; hence there was a large increase of induced abortions and a sharp decrease in the birth rate in 1966.

Instead of taking the risk of raising a “Fire Horse Woman,” whose headstrong nature would bring bad luck for her future husband, many Japanese families avoided having children entirely in 1966. In essence, superstition was embedded so deeply in Japanese culture that we could measure its effects on a macro-population scale.

Time will tell if superstition will strike again 10 years from now in 2026, the next year of the “Fire Horse” in its 60-year cycle. Given that Japanese is already below the replacement fertility rate (i.e., roughly an average of 2 children per woman), the result could be disastrous.