Test-retest reliability — establishing that measurements remain consistent across multiple testing sessions — is critical to measuring, understanding, and predicting individual differences in infant language development. However, previous attempts to establish measurement reliability in infant speech perception tasks are limited, and reliability of frequently-used infant measures is largely unknown. The current study investigated the test-retest reliability of infants' preference for

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... cted speech (hereafter, IDS) over adult-directed speech (hereafter, ADS) in a large sample (N=158) in the context of the ManyBabies1 collaborative research project (hereafter, MB1; Frank et al., 2017; ManyBabies Consortium, 2020). Labs of the original MB1 study were asked to bring in participating infants for a second appointment retesting infants on their IDS preference. This approach allows us to estimate test-retest reliability across three different methods used to investigate preferential listening in infancy: the head-turn preference procedure, central fixation, and eye-tracking. Overall, we find no consistent evidence of test-retest reliability in measures of infants' speech preference (overall r = .09, 95% CI [-.06,.25]). While increasing the number of trials that infants needed to contribute for inclusion in the analysis revealed a numeric growth in test-retest reliability, it also considerably reduced the study's effective sample size. Therefore, future research on infant development should take into account that not all experimental measures may be appropriate for assessing individual differences between infants.