Trivers R L, Willard D E
Science. 1973 Jan 5;179(4068):90-2. doi: 10.1126/science.179.4068.90.
Theory and data suggest that a male in good condition at the end of the period of parental investment is expected to outreproduce a sister in similar condition, while she is expected to outreproduce him if both are in poor condition. Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. Data from mammals support the model: As maternal condition declines, the adult female tends to produce a lower ratio of males to females.
理论和数据表明，在亲代投资期结束时状况良好的雄性预计比处于类似状况的雌性繁殖能力更强，而如果两者状况都不佳，雌性预计比雄性繁殖能力更强。因此，自然选择应有利于亲代根据自身投资能力来调整后代的性别比例。来自哺乳动物的数据支持这一模型：随着母体状况下降，成年雌性所产雄性与雌性的比例往往会降低。
