Argumentum ad temperantiam

An argumentum ad temperantiam, also known as a false compromise or as taking the middle ground, is a logical fallacy in which a position between two extreme points of view is promoted as the correct position. It is considered a fallacy because the quality of being a compromise has nothing to do with the veracity of a claims. A compromise may itself have implications which require scrutiny.

Example
The "middle ground" is most often invoked when there are sharply contrasting views which are deeply entrenched.
 * 1) God created humans in their present form six to ten thousand years ago.
 * 2) Humans evolved from ape-like ancestors over millions of years through a purely natural process.
 * 3) Middle ground: Humans evolved over a long time, but that process was initiated and guided by God.

Why this is a fallacy
Consider the following, more tongue-in-cheek examples:
 * A math lesson:
 * Teacher: What is 17 plus 18?
 * Student A: 25.
 * Student B: No, it's 35.
 * Teacher: Ah, then the correct answer must be around 30.
 * A tough choice:
 * Inquisitor: Would you rather be thrown into boiling water or molten lava?
 * Prisoner: I don't like either option. How 'bout something in between, like boiling oil?
 * From the Old Testament:
 * When King Solomon was presented with two women who both claimed to be the mother of a baby, he proclaimed that the baby should be cut in two, and half given to each woman. As a method of determining which one was telling the truth, it worked perfectly (one woman protested and gave up her claim to the child to spare its life, thus revealing herself as the real mother), but Solomon's decree can be seen as an example of falsely believing that the "middle ground" (between giving all of the baby to one woman or the other) is necessarily best for all parties involved.

Averages
A superficially similar idea to "the truth lies somewhere in the middle" is the idea that an average of many numerical observations will likely be closer to the true value than an individual observation. See Law of large numbers for more information and Law of averages for a related misconception.