Does the decision of the US Supreme Court have to be unanimous?

No. Sometimes verdicts are unanimous, but more often they are split down political and ideological lines. The Supreme Court has become increasingly polarized in the 20th and 21st Centuries, making unanimous decisions the exception, rather than the rule. On the Roberts' Court, many decisions are made by a vote of 5-4, with the conservatives and progressives unable to agree on many issues. Justice Anthony Kennedy, who has a mixed voting record, is typically the swing vote.

The Supreme Court requires only a simple majority vote to decide a case: If all nine justices are present, five of the nine must agree. The Court must have a quorum of six justices to hear a case.

YES, depending on the term. For the Court terms 2002-2014, Slate's Eric Posner reports that the Supreme Court handed down unanimous decisions in about half the cases they decided, while close 5-4 decisions were about 20% of the decisions. Fragmentation on the Supreme Court (close 5-4 decisions) was higher in the 1980's and before, but since about 1995 the fragmentation has been decreasing.

YES. Over the last sixty years, unanimous decisions have been almost TWICE as common as close 5-4 decisions. In the Roberts Court (2005 - present), unanimous decisions have been delivered by the Court in about half the cases decided. Reported by Slate's Eric Posner (Professor of Law at University of Chicago), and brenbarn.net