The idea that some groups of people may be more intelligent than othersis one of those hypotheses that dare not speak its name.
人类某些群体可能比其他群体更聪明,这一观点仍然是一种不敢冠之以名的假设。
句子的主干是The idea thatis one of those hypotheses..。第一个that引导同位语从句some groups of people may be more intelligent than others,解释Theidea的内容,第二个that引导定语从句,修饰oneofthosehypotheses.