Final answer:
The decision was to reject the null hypothesis based on the p-value being less than 0.05, indicating the results are statistically significant.
Step-by-step explanation:
When a researcher reports t(12) = 2.86, p < .05 for a repeated-measures research study, the implied decision is to reject the null hypothesis. This is because the reported p-value is less than the chosen alpha level of significance (typically alpha = 0.05 in social sciences). The decision to reject the null hypothesis is based on the fact that the p-value represents the probability of obtaining the observed results, or more extreme, if the null hypothesis were true. Here, a p-value smaller than the alpha level indicates that the observed data are unlikely under the null hypothesis and thus we reject it in favor of the alternative hypothesis.