This study aimed to systematically investigate the relationship between bilingualism, age, L2 onset age of acquisition (AoA), and white matter integrity (operationalized as fractional anisotropy, FA), addressing inconsistencies in the literature. We conducted a meta-analysis of 23 studies and used meta-regression models to assess the influence of age and L2AoA on effect sizes in studies comparing monolinguals and bilinguals. Even though the overall between-group effect size across the whole brain was unreliable, bilingualism was associated with increased white matter integrity in specific tracts and in groups with a limited range of age and L2AoA. Age had a small, negative effect on white matter integrity, with differences between monolinguals and bilinguals more pronounced in younger adults, consistent with a view of an initial increase in white matter integrity, followed by remodeling for efficiency over time. In contrast, later L2AoA was associated with greater white matter integrity in bilinguals than monolinguals, again consistent with the remodeling for efficiency model. Our findings highlight the importance of considering age and L2AoA when examining the neural basis of bilingualism on white matter in the brain and how bilingualism contributes to structural changes that stave off cognitive decline in older age.

本研究旨在系统地研究双语、年龄、第二语言习得起始年龄 (AoA) 和白质完整性（可操作为分数各向异性，FA）之间的关系，解决文献中的不一致之处。我们对 23 项研究进行了荟萃分析，并使用荟萃回归模型来评估年龄和 L2AoA 对单语者和双语者比较研究中效应大小的影响。尽管整个大脑的总体组间效应大小并不可靠，但双语与特定脑束以及年龄和 L2AoA 范围有限的组中白质完整性的增加有关。年龄对白质完整性有很小的负面影响，单语者和双语者之间的差异在年轻人中更为明显，这与白质完整性最初增加、随后随着时间的推移进行效率重塑的观点一致。相比之下，后来的 L2AoA 与双语者比单语者更大的白质完整性相关，这再次与效率模型的重塑一致。我们的研究结果强调了在研究双语对大脑白质的神经基础以及双语如何有助于避免老年认知能力下降的结构变化时考虑年龄和 L2AoA 的重要性。