You would need to have an increasing level of soy consumption in Japan for that to make sense as a suggested causal relationship. Simply "they eat a lot of soy

You would need to have an increasing level of soy consumption in Japan for that to make sense as a suggested causal relationship. Simply "they eat a lot of soy" and "their birth rate is declining" doesn't imply any kind of causation, if they were eating the same amount before it started declining.