Why is it so bad to be born in the Year of the Fire Horse?

Question

Japan's birth rate dropped precipitiously in 1966, only to recover completely in 1967.

This is apparently because 1966 was the "Year of the Fire Horse" - that is, in the 60-year zodiacal cycle you get by crossing the twelve zodiac animals with the five traditional elements (fire, water, earth, wood, metal), 1966 was in the Horse Year of the Fire cycle - and women actively avoided having children in that year because children born in that year were said to have poor prospects for marriage.

Is there some sort of mythological story explaining why it's so bad to be born in the Year of the Fire Horse?

Answer 1

1966 was a hinoe-uma (丙午) year. This does not come from "crossing zodiac with elements" per se, but is instead a specific combination of celestial stem and earthly branch in the traditional sexagenary dating system.

Both the stem hinoe (丙) and branch uma (午) belonged to the element of fire. This overabundance of fieriness informs the astrological interpretations that those born on this year have a fiery character - impulsive, vociferous, quick to anger. To the Japanese in particular, hinoe uma is believed to a year of disasters due to its (somewhat irrational) association with great fires.

Specifically, there is a traditional superstition in Japan that women born in the year of hinoe-uma would shorten their husbands' life (and presumably vice versa but not quite as oft-repeated). This interpretation seems motivated (with little basis) by the great fires of Edo, and was popularised along with such tales as that of attempted arsonist Yaoya Oshichi.

Oshichi had a crush on someone she met during a previous fire, and attempted to set another fire in order to meet him again. Notably, she is believed to have been born in 1666, a hinoe-um year.

Answer 2

There is some science backing this. In 1966 Japan began testing birth control and abortion which led to the decline of births. However they did this even more with women because there is an old omen that says that women born on this year would be impossible to marry off.

The paper "Increased induced abortion rate in 1966, an aspect of a Japanese folk superstition", published in the April 1975 issue of Annals of Human Biology discusses this:

A decrease of about 463 000 live births occurred in Japan in 1966, constituting a sharp departure from the linear trend before and after. This was partly caused by contraception and partly by induced abortion. The induced abortion rate, 43.1 per 1000 births in the year (a total of 65 000), was significantly higher than the 30.6 expected (46.200 total) from the regression trend computed from the years 1963 to 1969. No epidemics were reported in 1966 which might have caused the increase in abortion. It is more likely to be due to observance of Hinoe-Uma (Elder Fire-Horse), which comes round every sixty years by zodiac almanac. This event represents a superstition observed only by the Japanese, in which it is a bad omen for female babies to be born in the year. 1966 was the most recent year of Hinoe-Uma.

This led to the abortion rate of women to almost double this year.