Why does the question 'same birth time, same fate' matter to Vietnamese chart readers?
In Tu Vi, two people born in the same hour, day, month, and year receive the same chart. In Bat Tu, they receive the same four pillars. This is not rare: thousands of people are born in any given hour. If a chart determines destiny, those people should resemble each other to some degree, at least more than people born in other hours. That question can be put to data.
One line of testing has been done in the West with astrology, called the 'time twins' study. The idea is simple: take people born minutes apart, treat them as sharing the same astrological 'chart', then measure whether they resemble each other above chance in personality and life. If astrology is right, the similarity should be high. If not, the similarity should be no greater than for ordinary people.
What did the Dean and Kelly 2003 study do, and what did it find?
Geoffrey Dean and Ivan W. Kelly published in 2003 in the Journal of Consciousness Studies a review of astrological tests, including the London time twins study. The dataset covered more than two thousand people born in London in 1958, followed for decades across dozens of measures of personality, health, education, occupation, and life. The method compared pairs born minutes apart (effectively sharing a 'chart') with pairs born far apart, to see whether the 'same chart' group resembled each other more.
Kind of knowledge: UnprovenNot enough evidence. Someone has claimed it, but no source or study is strong enough to confirm it. The five labelsThe result: none of the similarity astrology predicts. In other words, being born at the same moment did not produce resemblance in personality or life path at the level astrological theory requires. This is one of the large-scale tests with long-term follow-up, so it is regarded as strong evidence in research on astrology.
| Study | Data scope | Main result |
|---|---|---|
| Dean & Kelly 2003, London 1958 time twins study | Over two thousand people born in London in 1958, followed for decades, dozens of measures | No similarity at the level astrology predicts |
| Hartmann et al. 2006 | Large sample, testing links between birth date and individual differences in personality and intelligence | Birth date does not predict personality or intelligence |
| Kelly 1998, Why Astrology Doesn't Work | Overview of astrological tests | The astrological community builds rescue hypotheses that make the system closed to negative evidence |
Why is long-term follow-up data suited to this test?
If birth timing determines personality and destiny, the effect must show up in measurable indicators that are stable over time. Long-term follow-up data allows comparison of many facets of the same person across years, reducing the chance of error from a single measurement. It also allows comparison between groups born close together and far apart, which is a direct test of the hypothesis that 'same birth moment means same fate'.
The strength of the time twins design is that it does not require participants to believe or disbelieve. It uses only objective data: birth date and time, and indicators measured over time. If astrology is right, the resemblance must appear whether or not people believe.
How do practitioners usually respond, and why is that a rescue hypothesis?
Many astrologers hold that a chart does not manifest identically in everyone because other factors intervene: the person's level of self-awareness and psychological maturity, biological factors such as age and sex, social factors such as economic and cultural position, and the person's own free choices. Kelly 1998 listed these arguments in 'Why Astrology Doesn't Work' and called them 'astrologers' excuses'. On Kelly's analysis, when a test fails, a new factor is added to explain it, making the system ever harder to falsify. This is what Imre Lakatos described as a 'degenerating research programme': instead of offering new testable predictions, people patch the theory after failure.
Fairness requires saying: some factors Kelly lists have a basis. For example, psychological and social factors really do affect how a person expresses personality. But that does not automatically save the astrological hypothesis, because if every factor can explain every result, the theory loses its capacity to be tested.
What did the study not test?
The Dean and Kelly 2003 study used Western astrology, not Tu Vi or Bat Tu. This must be said plainly. The 'no similarity' result applies to the Western astrological way of dividing the sky into signs and planets, not directly to the Tu Vi way of placing stars by can chi and birth hour.
For Tu Vi and Bat Tu, the question 'do people sharing a chart resemble each other' is a reasonable one, testable by similar methods, but to date no systematic study has done so. No study in the literature tests people sharing a Tu Vi chart. In Vietnam, birth records often lack an accurate birth hour, so testing faces a major data obstacle.
Other results point the same way
Kind of knowledge: UnprovenNot enough evidence. Someone has claimed it, but no source or study is strong enough to confirm it. The five labelsPeter Hartmann and colleagues published in 2006 in Personality and Individual Differences a large-scale study testing links between birth date and individual differences in personality and intelligence. The result: birth date does not predict personality or intelligence. This study did not test astrology directly, but it tested a foundational assumption of astrology: that birth timing is linked to psychological traits. The negative result here reinforces the Dean and Kelly conclusion.
Gerhard Mayer 2020 should also be mentioned: in a paper on the relationship between astrology and science, he reviewed the tests and noted that many placed astrologers' claims in difficulty, while citing Kelly's argument about a 'closed system'. Kind of knowledge: School: Geoffrey CorneliusSchool's interpretation. How one school or one book reads it; other schools may read it differently. The five labelsMayer also noted that some astrologers, such as Geoffrey Cornelius, hold that a psi factor (extrasensory capacity) could be the mechanism that lets astrology 'work' even when a chart is calculated wrongly. This is a minority hypothesis, not independently verified, and mainstream researchers see it differently.
A real effect that arises from belief
One real effect must be stated correctly: belief can change behaviour, and behaviour changes outcomes. If a person believes their chart says not to sign a contract today, they may postpone, and that postponement may lead to a different outcome. If a person believes they are the strong type, they may act more forcefully. This is an ordinary psychological effect, not evidence that the system predicts correctly. In other words, belief produces outcomes, but those outcomes do not confirm the system's predictive power.
So what is the answer for Vietnamese chart readers?
Kind of knowledge: UnprovenNot enough evidence. Someone has claimed it, but no source or study is strong enough to confirm it. The five labelsAvailable data do not support the idea that birth timing determines personality or life. The London 1958 time twins study showed that people born minutes apart did not resemble each other above chance on the measures taken. The Hartmann et al. 2006 study likewise showed that birth date does not predict personality or intelligence.
But a clear distinction is needed: these studies test Western astrology, not Tu Vi or Bat Tu. For Tu Vi and Bat Tu, the question of 'people sharing a chart' remains open, with no systematic study. In Vietnam, birth records lacking an accurate birth hour are a major obstacle. So a chart reader may keep their belief, but should know that available scientific evidence has not confirmed the hypothesis that 'birth timing determines destiny', and that no study has directly tested Tu Vi or Bat Tu in a similar way.
