Short Answer: How a Birth-Year Belief Once Shifted a Nation's Population
Kind of knowledge: Folk beliefFolk belief. What people believe, avoid or pass on by word of mouth, recorded as a belief, not as a checked fact. The five labelsIn the 1966 Bính Ngọ year, Japan recorded about a 15% drop in births compared to neighboring years. The cause was not epidemic or war, but a folk belief: girls born in the Bính Ngọ year (hinoeuma in Japanese) were thought to harm husbands and bring misfortune to the husband's family. Many couples actively avoided conception that year, shifting births to adjacent years, and a sizable share even selected fetal sex to avoid having daughters.
In South Korea, Horse years (1978, 1990, 2002) also showed lower fertility and higher sex ratios at birth, especially from the late 1980s when ultrasound and sex-selection techniques became widespread. These are the two countries with enough data for analysis. For Vietnam, 2026 is a Bính Ngọ year, but no research has measured the impact of Horse-year beliefs on reproductive behavior. This article does not extrapolate from Japan or Korea to Vietnam.
Bính Ngọ in Japanese Folk Belief and the Mechanism of Impact
Kind of knowledge: Folk beliefFolk belief. What people believe, avoid or pass on by word of mouth, recorded as a belief, not as a checked fact. The five labelsIn Japanese folk belief, women born in the Bính Ngọ year were thought to have unfavorable marital fates, often described as "harming the husband," "killing the husband," or bringing disaster to the husband's family. This belief is not an orthodox doctrine of any school, but an oral tradition deeply embedded in Japanese folk life since the Edo period onward.
Research by Chris Rohlfs and colleagues (2010) in the Journal of Development Economics quantified the impact of this belief. The authors found that Japan's births in 1966 fell about 15% below the general trend. Notably, they separated two mechanisms: complete avoidance of birth (without sex selection) and avoidance with sex selection. In other words, some families simply did not have children that year, while others still gave birth but intervened to ensure the child was a boy.
Why Would People Shift Births to Another Year?
Kind of knowledge: SchoolSchool's interpretation. How one school or one book reads it; other schools may read it differently. The five labelsShifting births is rational behavior within the belief framework. If a couple believes a girl born in the Bính Ngọ year will face misfortune, they may choose not to conceive that year, or if already pregnant, seek to deliver earlier or later. Demographically, this creates a "trough" in births in 1966 and a compensating "peak" in 1965 or 1967.
Rohlfs and colleagues (2010) also showed the effect was not just avoiding daughters, but substitution across birth years. Families who did not give birth in 1966 may have given birth in 1967, raising 1967 births. This is a form of "substitution across birth years" the authors describe in their paper's title.
Does the Belief Follow Migrants?
Research by K. Kaku and Y. S. Matsumoto (1975) in the American Journal of Public Health compared 1966 fertility among Japanese descendants in California and Hawaii with other populations. Results showed the Bính Ngọ belief affected not only those living in Japan, but also followed Japanese migrants to the US. This suggests cultural beliefs can persist independently of the host country's legal and health environment.
Kind of knowledge: UnprovenNot enough evidence. Someone has claimed it, but no source or study is strong enough to confirm it. The five labelsHowever, note this study only compared fertility rates, not directly measuring individual beliefs. Lower fertility among Japanese descendants in the US in 1966 could reflect the belief persisting in the community, or indirect influence from Japanese community opinion.
Did Women Born in 1966 Suffer Disadvantages?
Research by Hiroyuki Yamada (2012, published in Review of Economics of the Household) examined whether women born in the 1966 Bính Ngọ year actually faced disadvantages in education and marriage. Results showed no evidence of disadvantage in these indicators. In other words, the belief about the "fate" of those born in the Bính Ngọ year is not confirmed by actual data on their education and marriage.
This is an important point: the demographic effect is real, but the effect on those born that year is not seen. The belief changed parents' reproductive behavior, but did not create differences in life outcomes for those born that year.
So Is the Demographic Effect the System "Predicting Correctly"?
No. Distinguish clearly: the belief changed behavior (many avoided having daughters in the Bính Ngọ year), and behavior changed demographic outcomes (births fell). This is a real effect, but born from belief, not from any chart or system "predicting correctly" the fate of those born that year. Yamada's (2012) research shows those born in 1966 faced no actual disadvantage, further reinforcing that the demographic effect came from parents' behavior, not the child's "destiny".
South Korea 1970 to 2003: Horse Years and Sex Selection
Research by Jungmin Lee and Myungho Paik (2006) in the journal Demography analyzed South Korean data from 1970 to 2003. The authors found that in Horse years, the sex ratio at birth (boys per 100 girls) rose, and fertility fell. The effect was stronger from the late 1980s, when ultrasound and fetal sex-selection techniques became widespread.
Kind of knowledge: SchoolSchool's interpretation. How one school or one book reads it; other schools may read it differently. The five labelsThis shows a different mechanism from Japan: in South Korea, instead of avoiding birth entirely, many families chose to have sons in Horse years to avoid daughters. The belief in South Korea may not be exactly like Japan's, but it also led to sex-selective behavior.
Lee and Paik (2006) also noted the effect was not limited to Horse years, but could relate to other years in the 12-animal cycle. However, the Horse year was the most thoroughly analyzed in their study.
Fetal Sex Selection Is Banned by Vietnamese Law
In Vietnam, fetal sex selection is banned by law. This means if Horse-year beliefs exist in Vietnam, the mechanism would differ from South Korea, since sex selection cannot be legally intervened.
How Beliefs Spread and Fade: Cultural Evolution Models
Research by Cinthia Marie Tanaka and Yoh Iwasa (2012) in the Journal of Theoretical Biology built a dynamic model of the spread of the Bính Ngọ belief in Japan. Their model examined whether the belief is transmitted through mothers or fathers, and how it can spread or decline in a population.
Kind of knowledge: UnprovenNot enough evidence. Someone has claimed it, but no source or study is strong enough to confirm it. The five labelsModel results showed the belief can spread through both genders, but the rate depends on how it is transmitted in family and community. This is a theoretical study, not directly measuring real populations, but it provides a framework explaining why the belief can persist across generations despite no biological basis.
Can the Belief Fade?
Kind of knowledge: UnprovenNot enough evidence. Someone has claimed it, but no source or study is strong enough to confirm it. The five labelsStudies on Japan show that after 1966, the Bính Ngọ belief persisted but its demographic impact gradually declined in subsequent Bính Ngọ years (1906, 1966, 2026). This may be due to many factors: urbanization, education, changing views on marriage, and general fertility decline. However, no study in available sources systematically measures this decline for 2026.
Vietnam in 2026: No Data, No Predictions
Kind of knowledge: UnprovenNot enough evidence. Someone has claimed it, but no source or study is strong enough to confirm it. The five labelsNo published research exists on the impact of Horse-year beliefs on reproductive behavior in Vietnam. Nor is there 2026 data since the year has not ended. This article makes no forecasts for Vietnam.
Kind of knowledge: SchoolSchool's interpretation. How one school or one book reads it; other schools may read it differently. The five labelsSome differences between Vietnam and Japan, South Korea could affect outcomes if future research is done. First, Vietnam bans fetal sex selection, so the sex-selection mechanism would not match South Korea. Second, Horse-year beliefs in Vietnam may differ from Japan (where the belief focuses on girls) and South Korea (where it may involve boys). Third, fertility in Vietnam has declined in recent decades, so the impact of a single belief may be harder to isolate.
Comparison Table of Studies
| Study | Country | Period | Main Findings |
|---|---|---|---|
| Rohlfs et al. (2010) | Japan | 1966 | Births fell about 15%; separated avoidance without sex selection and with sex selection |
| Kaku and Matsumoto (1975) | US (Japanese descendants in California and Hawaii) | 1966 | Compared 1966 fertility among Japanese descendants |
| Yamada (2012) | Japan | 1966 | Women born in Bính Ngọ year faced no disadvantage in education, marriage |
| Lee and Paik (2006) | South Korea | 1970-2003 | Horse years: sex ratio at birth rose, fertility fell |
| Tanaka and Iwasa (2012) | Japan (theoretical model) | Not applicable | Model of Bính Ngọ belief transmission through mother or father |
What Stands Out from These Studies?
These studies show birth-year beliefs can create measurable demographic effects, but the effect does not confirm the belief's content. In other words, lower births in the Bính Ngọ year do not mean girls born that year actually harm husbands. Yamada's (2012) research shows those born in 1966 faced no actual disadvantage.
Also note all studies above were conducted in Japan, South Korea, or Japanese-descendant communities in the US. No study exists on Vietnamese people. So it cannot be inferred that the 2026 Bính Ngọ year in Vietnam will have similar effects.
On Terminology
In this article, terms like "Bính Ngọ," "Horse year," "hinoeuma" are kept as in Vietnamese and Japanese usage, not transliterated into Chinese. "Hinoeuma" is the Japanese term for the Bính Ngọ year, used in studies on Japan.
Conclusion
Birth-year beliefs once shifted a nation's population. In Japan in 1966, births fell about 15% due to the belief that girls born in the Bính Ngọ year harm husbands. In South Korea, Horse years were linked to higher sex ratios at birth and lower fertility. But those born that year showed no disadvantage in education or marriage. This is a lesson in how belief can change behavior without being factually true. For Vietnam in 2026, there is no data, and this article offers no forecast.
