This article compiles results from a century of social psychological research, including more than 25,000 studies involving 8 million people. It presents meta-analytic information on the magnitude and variability of social psychological effects, referencing 322 meta-analyses and statistical effect-size summaries. The analysis reveals that social psychological effects typically yield a value of r equal to .21, with a standard deviation in r of .15 across studies. The article discusses the uses, limitations, and implications of this large-scale compilation. In 1898, Norman Triplett published an early experiment in social psychology, examining the effect of the presence of others on task performance. Over the past century, many social psychological effects have been documented. This article summarizes the best-established findings, with data from more than 25,000 studies. The goal is to quantify the magnitude and variability of social psychological effects. The article reviews previous summaries of social psychology, notes unresolved issues, and discusses developments that allow a century of scholarly work to be quantitatively described. The article discusses the size and consistency of social psychological effects, noting that some believe many effects are small, while others report larger effects. The article also addresses the replicability of social psychological effects, noting that some believe they are nonreplicable, while others argue they are stable. These contentions are important for the scientific status of the field. The article discusses the use of meta-analysis in social psychology, which requires a large-scale compilation of evidence. Meta-analysts convert effect sizes to a common metric, such as a Pearson product-moment correlation coefficient (r), and compute a weighted mean effect size. They also examine the heterogeneity of effects in a research literature. The article presents a quantitative summary of a century of social psychological research, compiling quantitative reviews published before 1998. It seeks to compile quantitative reviews of social psychological topics and provides summaries of the mean size of the effect in each research literature and the variability in effect sizes across studies. The article discusses the methods used to retrieve and select documents for the compilation, including searching databases, examining special journal issues, and consulting lists of meta-analyses. The criteria for inclusion were that the document had to report a numerical measure of the combined magnitude or significance level of a relationship between two variables measured on individuals or small groups. The article discusses the selection and coding of effects, including the mean effect size, corrected standard deviation, and other variables. It also discusses additional coding of effects, including the number of primary effect-size estimates, the proportion of unpublished research, and the number of theoretical and artifactual moderator analyses. The results show that the mean size of social psychological effects is .21, with a median of .18 and a standard deviation of .15. The article discusses the magnitude of social psychological effects, noting that many effects are small, but some are larger. It also discusses theThis article compiles results from a century of social psychological research, including more than 25,000 studies involving 8 million people. It presents meta-analytic information on the magnitude and variability of social psychological effects, referencing 322 meta-analyses and statistical effect-size summaries. The analysis reveals that social psychological effects typically yield a value of r equal to .21, with a standard deviation in r of .15 across studies. The article discusses the uses, limitations, and implications of this large-scale compilation. In 1898, Norman Triplett published an early experiment in social psychology, examining the effect of the presence of others on task performance. Over the past century, many social psychological effects have been documented. This article summarizes the best-established findings, with data from more than 25,000 studies. The goal is to quantify the magnitude and variability of social psychological effects. The article reviews previous summaries of social psychology, notes unresolved issues, and discusses developments that allow a century of scholarly work to be quantitatively described. The article discusses the size and consistency of social psychological effects, noting that some believe many effects are small, while others report larger effects. The article also addresses the replicability of social psychological effects, noting that some believe they are nonreplicable, while others argue they are stable. These contentions are important for the scientific status of the field. The article discusses the use of meta-analysis in social psychology, which requires a large-scale compilation of evidence. Meta-analysts convert effect sizes to a common metric, such as a Pearson product-moment correlation coefficient (r), and compute a weighted mean effect size. They also examine the heterogeneity of effects in a research literature. The article presents a quantitative summary of a century of social psychological research, compiling quantitative reviews published before 1998. It seeks to compile quantitative reviews of social psychological topics and provides summaries of the mean size of the effect in each research literature and the variability in effect sizes across studies. The article discusses the methods used to retrieve and select documents for the compilation, including searching databases, examining special journal issues, and consulting lists of meta-analyses. The criteria for inclusion were that the document had to report a numerical measure of the combined magnitude or significance level of a relationship between two variables measured on individuals or small groups. The article discusses the selection and coding of effects, including the mean effect size, corrected standard deviation, and other variables. It also discusses additional coding of effects, including the number of primary effect-size estimates, the proportion of unpublished research, and the number of theoretical and artifactual moderator analyses. The results show that the mean size of social psychological effects is .21, with a median of .18 and a standard deviation of .15. The article discusses the magnitude of social psychological effects, noting that many effects are small, but some are larger. It also discusses the