This paper examines the impact of sibling size on children's intellectual development and academic achievement through a critical analysis of two opposing theories：Resource Dilution Theory and the Confluence Model. Downey's article supports Resource Dilution Theory，highlighting the negative consequences of larger families on educational outcomes due to resource dilution. Phillips' article critiques Guo and VanWey's research，which challenges resource dilution theory and argues for the importance of parental investment and individual differences. The paper concludes by emphasising the need for further research that takes into account these factors and the complexity of the relationship between sibling size and educational outcomes.