When one think of tests of statistical significance, hypotheses must be considered first. With non-directional null hypotheses, you are only suggesting that there will be a difference in mean score results. Which of the means will be higher is not speculated upon. In the case of a non-directional null hypothesis, a two-tailed test of significance is used.
If there is a prediction of which mean will be higher and which will be lower, then a one-tailed test of significance is used. When you state a directional hypothesis, a one-tailed test of significance is used.
Non-directional null hypothesis = Two-tailed test of significance
Directional null hypothesis = One-tailed test of significance
Return from tests of statistical significance to dissertation research questions and hypotheses.