Understanding the proper conditional relationship represented by the “either/or” conjunction in LSAT questions is crucial in both Logic Games and Logical Reasoning questions. While there are some solid rules you must follow, ultimately you should take into account the context in which the phrase is used.
Typically, a rule such as “either A or B must be selected” is inclusive: it allows for the possibility of selecting both A and B (unless the rule specifically precludes that by stating “either A or B must be selected, but not both”). Thus, at least one of A or B must be selected:
However, compare this rule to the following:
Mary has a higher LSAT score than does either Kate or Jane.
Whenever this wording is used (“than does either” or “than either”), we can infer that Mary’s LSAT score is higher than either of the other two girls’: i.e. Mary has a higher score than both of them. In this case, “either” requires the use of the conjunction “or” to make the sentence is grammatically correct.
Let's take a few examples.
In the statements below, “either...or” means “both”:
Anne arrives earlier than does either Bob or Cathy.
Jack is a better chess player than is either his mentor, or his opponent.
By contrast, in the following statements “either/or” does not necessarily mean “both,” and just means “at least one”:
Anne arrives before Bob or before Cathy.
Jack is either a better chess player than his mentor, or a better chess player than his opponent.
Finally, in the statements below, “either/or” precludes the possibility of “both”:
Anne arrives before either Bob or Cathy, but not before both.
Jack is either a better chess player than his mentor, or a better chess player than his opponent, but Jack is not a better player than both his mentor and his opponent.
The point of this discussion is that you must be very careful when you see the “either/or” construction, and you cannot assume that is always means “at least one, possibly both.” As we have seen, there are certain circumstances when it means just “both,” and there are times when the test makers throw in additional language to eliminate the possibility of “both.” Always read very closely, and you will be rewarded when the test makers go beyond the normal usage!