Social investment theory (SIT) proposes that the transition to parenthood triggers positive personality trait change in early adulthood. Using data from a representative sample of first-time parents compared to nonparents, the results of rigorous tests do not support the propositions of SIT. Specifically, we found no evidence for the proposition that parents show more pronounced mean-level increases in emotional stability, agreeableness, and conscientiousness compared to nonparents. We did find that agreeableness and openness changed depending on how long someone was in the parent role. Finally, our results suggest that high extraversion and low openness in both genders and high conscientiousness in females predict the likelihood to enter into parenthood. Discussion focuses on why this transition seems to be unrelated to mean-level personality trait change and the implications of these results for SIT.