本文提出了儿童发展的经济模型，这些模型捕捉了最近从技能形成的经验文献中发现的精髓。本文的目标是提供一个理论框架，以解释来自大量实证文献的证据，指导下一代实证研究，并制定政策。我们分析的核心是一个概念，即童年不止一个阶段。我们将自我生产力和人力资本投资互补性的概念正式化，并用它们来解释技能形成的证据。总之，他们解释了为什么技能通过乘数过程产生技能。技能形成是一个生命周期过程。它始于子宫，贯穿一生。在这个过程中，家庭所起的作用远比学校的作用重要。成人的成功需要多种技能和能力。能力既是遗传的，也是创造的，传统的关于先天和后天的争论在科学上已经过时了。 Human capital investment exhibits both self-productivity and complementarity. Skill attainment at one stage of the life cycle raises skill attainment at later stages of the life cycle (self-productivity). Early investment facilitates the productivity of later investment (complementarity). Early investments are not productive if they are not followed up by later investments (another aspect of complementarity). This complementarity explains why there is no equity-efficiency trade-off for early investment. The returns to investing early in the life cycle are high. Remediation of inadequate early investments is difficult and very costly as a consequence of both self-productivity and complementarity.