Two-tailed tests are more conservative and less powerful than one-tailed tests.
In statistics, one-tailed and two-tailed tests test are used to determine the statistical significance of a variable on a sample of people. Two-tailed tests are more conservative than one-tailed tests because by using a two-tailed test, you are determining the possibility of a causality between variables in both directions, unlike a one-tailed test where you are determining the possibility of a causality between variables in one direction only.
Additionally, two-tailed tests are less powerful than one-tailed tests because a two -tailed test has its alpha value set at half the value of a one-tailed test (.10 for a two-tailed test and .05 for a one-tailed test).
