为解决老龄化、少子化和劳动力短缺等人口结构失衡的问题，我国于2016年和2021年相继放开了全面二孩和全面三孩政策，但并未达到预期效果，中国居民生育率持续下滑。担心生育更多子女会削减单个子女所获得的教育投入、进而产生“数量替代质量”的生育焦虑，是生育政策松动后中国居民生育率不升反降的重要原因。事实上，同胞数量对个体教育获得的影响既有资源竞争的一面，也有资源供给的一面，即通过同胞间的互动和支持所产生的积极溢出效应。换言之，同胞可能成为代内资源的供给者，并对个体的人力资本积累产生正向影响。

本文基于2010至2018年中国家庭追踪调查数据（China Family Panel Studies, CFPS），通过描述性分析、OLS回归和Probit回归等方法，以及倾向得分再加权、引入早期认知能力变量和工具变量法等因果推断策略，揭示了兄姐上大学对弟妹高等教育机会获得的溢出效应，剖析了同胞溢出效应的内在机制，以及识别了同胞溢出效应在不同社会阶层上的分化状况。本文主要得到以下结论：

第一，兄姐上大学对弟妹高等教育机会获得存在积极的溢出效应。（1）基于线性概率模型和Probit模型的基准回归结果显示，兄姐上大学会显著提升弟妹获得高等教育的机会。（2）上述结论在倾向得分再加权、引入早期认知能力变量和将大学扩招作为工具变量等因果推断策略的识别下同样成立。（3）变换样本的稳健性检验发现，无论是出生间隔最近的兄姐还是长兄长姐，均能提升弟妹未来上大学的可能性，且积极的同胞溢出效应稳定地存在于不同性别组合特征和不同年龄组合特征的同胞中；改变变量测量方式的稳健性检验发现，兄姐所上大学的层次对弟妹高等教育机会获得的层次也有很强的正向预测作用。

第二，角色榜样机制和情景效应机制在同胞溢出效应产生过程中发挥了重要传导功能，而社会互动机制并未得到支持。（1）基于自我教育期望变量，检验兄姐上大学可以通过积极榜样示范、提升弟妹教育期望的假定，结果发现兄姐上大学能够显著提升弟妹期望自己上大学的概率，角色榜样机制得到验证。（2）基于父母教育期望变量，检验兄姐上大学能够提升父母对后来子女的预期、改善弟妹所处养育情景的假定，结果发现兄姐上大学能够显著提升父母对弟妹的教育期望，情景效应机制得到支持。（3）基于自尊水平变量，检验兄姐上大学可能在同胞互动过程中降低弟妹自我认知和自尊心的假定，结果发现兄姐上大学对弟妹的自尊水平没有显著影响，社会互动机制并未得到验证。

第三，弱势阶层中的积极同胞溢出效应大于优势阶层。（1）按照城乡户籍状况对家庭经济地位进行划分，发现在低经济地位家庭中，兄姐上大学对弟妹高等教育机会获得的积极溢出效应更为强烈。（2）按照父母受教育水平对家庭文化地位进行划分，发现低文化地位家庭中的积极同胞溢出效应更加凸显。

本文为三孩政策实施后“想生”但又“不敢生”的父母减轻生育焦虑、以及为多子女家庭突破养育困境提供了新的思路。基于研究发现，本文认为，在当代中国的多子女家庭中，高等教育机会获得中的同胞竞争效应或许存在，但同胞支持效应也在不断彰显。积极的同胞溢出效应提高了家庭人力资本投资的效用，父母针对年长子女的教育投入同样会使年幼子女受益，并且这种提升效应在弱势家庭中更加凸显。

在实证分析的基础上，本文提出三点建议。第一，向社会大众传递生育多孩对子女人力资本积累存在积极影响的信息，缓解中国居民由于资源稀释效应而引发的生育焦虑。第二，多孩家庭的父母应重视对年长子女的培养，并注重在家庭中构建亲密的同胞关系、营造同胞互助的文化氛围，尽可能将积极的同胞溢出效应发挥至最大化，提高子代培养质量。第三，教育管理者可以基于代际教育和代内教育合力的角度，探索针对弱势家庭子代的家校合作新模式。

In order to address issues such as aging population, low birth rates, and labor shortages, China implemented the "universal two-child" and "universal three-child" policies in 2016 and 2021, respectively. However, these policies have not achieved the expected results, as the fertility rate in China has continued to decline. Concerns about reducing education investment per child and the resulting "quantity over quality" anxiety about having more children are important reasons for the failure of the relaxation of China's birth policies to increase the fertility rate. In fact, the impact of sibling number on an individual's educational attainment has both a resource competition aspect and a resource supply aspect, which is the positive spillover effect generated by interactions and support among siblings. In other words, siblings may become suppliers of intra-generational resources and have a positive impact on individual human capital accumulation.

Using China Family Panel Studies (CFPS) data from 2010 to 2018, this study employs descriptive analysis, OLS regression, Probit regression, and causal inference strategies such as propensity score weighting, introducing lagged ability variables, and instrumental variable methods. The study reveals the spillover effect of older siblings attending university on younger siblings' access to higher education opportunities, explores the underlying mechanism of sibling spillover effects, and identifies the differentiation of sibling spillover effects in different social strata. The main findings of this study are as follows:

First, siblings' college attendance has a positive spillover effect on their younger siblings' opportunities for higher education. (1) The regression results based on linear probability model and Probit model show that siblings' college attendance significantly increases their younger siblings' chances of obtaining higher education. (2) The above conclusion remains valid under causal inference strategies. (3) Robustness tests using different samples show that both the oldest siblings and older siblings with the closest age gap can increase the likelihood of their younger siblings attending college, and the positive spillover effect exists stably among siblings with different gender and age compositions. Robustness tests using different ways of measuring variables show that the level of college attendance of siblings has a strong positive predictive effect on their younger siblings' opportunities for higher education.

Second, the role model mechanism and the context effect mechanism play an important role in the process of sibling spillover effects, while the social interaction mechanism is not supported. (1) Based on the variable of self-education expectation, it is found that siblings' college attendance can significantly increase the probability of their younger siblings expecting to attend college, supporting the role model mechanism. (2) Based on the variable of parental education expectation, it is found that siblings' college attendance can increase parents' expectations for their younger siblings' education and improve the nurturing situation of their younger siblings, supporting the context effect mechanism. (3) Based on the variable of self-esteem level, it is found that siblings' college attendance does not significantly affect their younger siblings' self-esteem level, and the social interaction mechanism is not supported.

Third, the positive spillover effect among siblings is stronger in disadvantaged families than in advantaged ones. (1) According to the household economic status divided by urban and rural hukou, it is found that the positive spillover effect of siblings' college attendance on their younger siblings' opportunities for higher education is stronger in low-income families. (2) According to the household cultural status divided by parents' education level, it is found that the positive spillover effect among siblings is more pronounced in families with low cultural status.

This article provides new insights into reducing the anxiety of having multiple children and overcoming difficulties in raising them. Based on empirical research, this study suggests that while the competition effect among siblings may exist, the sibling support effect is also increasingly prominent. The positive sibling spillover effect enhances the effectiveness of human capital investment in families, as educational investment for older siblings can also benefit younger siblings, with this enhancement effect being more pronounced in disadvantaged families.

Building on these empirical findings, the article proposes three recommendations. First, disseminate information to the public about the positive impact of having multiple children on children's human capital accumulation to alleviate the anxiety caused by the dilution effect of resources in China. Second, parents of multiple children should attach importance to cultivating older siblings and pay attention to building a close sibling relationship and a culture of mutual assistance in the family, maximizing the positive sibling spillover effect and improving the quality of offspring education. Third, education managers can explore new models of home-school cooperation targeting offspring from disadvantaged families based on intergenerational and intra-generational education.