Commas before "Because"

Summary: you rarely need a comma before "because," even when the word connects two independent clauses. The reason for this is that using a comma reduces the causal relationship between the clauses.

Writer's tip: don't use a comma before “because,” unless the first clause contains a negative, or the second clause is nonessential.

See also: Using Commas With “As”

Explanation:

While "because" often connects two independent clauses, and we are taught to place a comma before conjunctions connecting independent clauses, there is a rule that supersedes this one and tells us not to use a comma with "because" (in 99% of cases).

The rule is as follows:

If the clause is nonessential, use a comma before it, and conversely, if the clause is essential, don't use a comma.

E.g. She was Terrance's girlfriend, the same girl George had seen eating with Mark on Sunday. ("She" was identified as Terrance's girlfriend, the fact that George saw her eating with Mark on Sunday is nonessential.)
With an essential clause, the information is necessary to understand the sentence, so no comma should separate it from the clause which preceded it.

E.g. She was the one who had eaten with Mark on Sunday. (Without the clause, "who had eaten with him on Sunday," we wouldn't know who "she" is, so the information is critical to the meaning of the sentence.)

By its very nature, "because" makes the clause that follows it essential (due to the causal relationship that the word implies), and the comma should be omitted.

Here is an example:

He went to bed because he was tired.

Technically, "He went to bed" and "he was tired" are both independent clauses because they each have a subject and a verb, but when you join them with "because," it would be incorrect to use a comma.

Some more examples:

There was nothing to eat because the cupboard was bare.

He always ate alone because he didn't like talking with his mouth full, and he hated cold food.

When a comma before "because" is necessary:

1. He didn't run because he was afraid. (Then why did he run?)

He didn't run, because he was afraid.

2. He wasn't happy because they lost. (Then why was he happy?)

He wasn't happy, because they lost.

3. She didn't like him because he was tall. (Then why did she like him?)

She didn't like him, because he was tall.

Notice that in each case, the version of the sentence without the comma seems to say the opposite of what the one with the comma does. Moreover, the versions without commas don't even seem like complete sentences; there seems to be a missing clause in each. But also note the pattern: the first clause in each case used a negative--"didn't" or "wasn't."

Therefore, here's a good writer's tip: don't use a comma before "because" when joining two independent clauses, unless the first clause contains a negative.

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