In all honesty, probably not that much different. If fertility rates in richer and less dictatorial Confucian societies are any indicator, China's birth rate would have fallen as drastically as it did, and in any case the One Child Policy doesn't apply to ethnic minorities (in fact, the TFR in one part of Tibet is still over five), or most rural areas if the first child was a girl. There would still be a frightening gender imbalance, and already many Chinese men are importing their brides from abroad.