This paper investigates the mechanisms underlying scientific stratification in the era of transition from elite to mass science. Existing scholarship has largely examined scientific stratification through the Matthew effect framework at the individual, institutional, and lineage levels, but this theoretical lens has grown limited in today's academic landscape, where mass, team-based, and lab-centered research has become the dominant mode of knowledge production. As scientists increasingly share institutional and lineage backgrounds, considerable variation within these units remains unexplained. We propose a new framework that integrates concepts and methodological tools from demography into the social study of science. Drawing on the parallel between biological families and scholarly lineages as fundamental units of reproduction, we adapt the concept of birth order to examine how the sequence of doctoral students within a lineage shapes their career trajectories. Using data on more than one million U.S. doctoral graduates, our analysis shows that, much like in biological families, later students systematically perform worse than earlier ones across multiple dimensions of academic achievement, both short and long term. Examining the underlying mechanisms, we find that later students receive less cognitive stimulation from mature scholars and instead more from peers, and specialize in narrower intellectual domains as senior siblings occupy adjacent territories. These factors constrain their intellectual development as independent scholars. By introducing a demographic framework into the study of science, this paper offers a new perspective on scientific stratification and demonstrates how demographic concepts can be fruitfully extended to analyze broader social and epistemic systems.

翻译：本论文研究从精英科学向大众科学转型时代科学分层的潜在机制。现有研究主要基于马太效应框架，从个人、机构与学术谱系三个层面探讨科学分层，但在当今学术环境中，大众化、团队导向与实验室中心的研究已成为知识生产的主导模式，这一理论视角存在局限。随着科学家共享机构与学术谱系背景的现象日益普遍，这些单元内部存在的显著差异尚未得到解释。我们提出一个新框架，将人口学概念与方法论工具整合至科学社会研究之中。借鉴生物学家族与学术谱系作为基本再生产单元的类比，我们将出生顺序概念进行适应性调整，以分析博士生在学术谱系中的顺位如何塑造其职业发展轨迹。基于逾百万美国博士毕业生的数据分析表明，与生物学家族高度相似的是，在学术成就的短期与长期多维度指标上，后入学学生的表现系统性地低于先入学学生。通过探究潜在机制，我们发现后入学学生从成熟学者处获得的认知刺激更少，更多依赖同侪互动，且由于资深学术同胞已占据邻近研究领域，其学术专攻范围更趋狭窄。这些因素制约了他们作为独立学者的智力发展。通过将人口学框架引入科学分层研究，本文为理解科学分层提供了新视角，并展示了如何将人口学概念有效延伸至分析更广泛的社会与认知系统。