Two-tailed t-tests are often used because any predictor that differs significantly from zero in a two-tailed test will also be significantly greater than zero or less than zero in a one-tailed test at the same α.

Question

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In a two-tailed test, the null hypothesis states that the predictor is equal to zero and the alternative hypothesis states that it is different from zero (either greater than or less than). In a one-tailed test, the null hypothesis states that the predictor is equal to or less than zero (for a one-tailed lower test) or equal to or greater than zero (for a one-tailed upper test). Therefore, if a predictor is significant in a two-tailed test, it will also be significant in a one-tailed test in the same direction, but not necessarily in the opposite direction.

Two-Tailed T-Tests Are Often Used Because Any Predictor That Differs