Given the legitimacy challenges faced by entrepreneurs, gaining access to the resources necessary to create viable new ventures is often difficult. Accordingly, scholars advocate that entrepreneurs align with high-status partners to convey that they are an accepted part of the sociocultural and organizational landscape. Although startup accelerators have been argued to play this supportive role for high-tech, high-growth ventures, it remains unclear whether they are effective at serving the needs of ventures pursuing social missions alongside business structures, or for-profit social ventures (FPSVs). To explore this issue, we examine whether social impact accelerators (SIAs), accelerators specifically designed to support FPSVs, help such ventures make the transition from mere ideas to viable organizations, a process known as emergence. To determine a causal relationship, we employ a quasi-experimental design and adopt propensity score matching with the nearest neighbor matching algorithm to study 7185 startups that applied to 383 accelerators worldwide from 2013 to 2019. By matching accepted startups to a control group of rejected startups, we find that SIAs, on average, facilitate new venture emergence, with accelerated FPSVs raising more external financing, earning more revenues, and hiring more full-time employees than their unaccelerated counterparts. These results hold when controlling for selection bias, thereby providing robust evidence for a causal relationship between acceleration and startup emergence. However, a subsequent subgroup analysis reveals that this causal effect is contingent across a breadth of "who," "when," and "where" contexts, highlighting the idiosyncratic differences that different startups face in the acceleration process. Plain English Summary: Recent empirical research on the benefits provided by startup accelerators suggests that accelerators help early-stage, commercial, growth-oriented ventures access external financing. While useful in demonstrating the value accelerators provide, we see an opportunity to expand this focus to explore the acceleration effect in a broader context. Given the important role social innovations play in the global economy, we explore how social impact accelerators (SIAs), entities designed to help startups simultaneously scale economic and social impact, help nascent for-profit social ventures (FPSVs) transform into viable, self-sustaining organizations, a process known as emergence. We find that SIAs do indeed facilitate emergence, with accelerated FPSVs raising more external financing, earning more revenues, and hiring more full-time employees than unaccelerated FPSVs. However, the SIA effect varies depending on how old the startup is, the gender composition of the founding team, and where the startup is located. By validating the causal impact of SIAs on FPSV emergence, our findings can inform not only theory and practice, but also public policy focused on bolstering the social innovation ecosystem. [ABSTRACT FROM AUTHOR]