Here's the relevant California statutes.
If both parents agree to 50/50 it is presumed to be in the best interests of the children. If one parents does not agree it defaults back to section 3011, better known as best interests of the children.
3080. There is a presumption, affecting the burden of proof, that
joint custody is in the best interest of a minor child, subject to
Section 3011, where the parents have agreed to joint custody or so
agree in open court at a hearing for the purpose of determining the
custody of the minor child.
3011. In making a determination of the best interest of the child
in a proceeding described in Section 3021, the court shall, among any
other factors it finds relevant, consider all of the following:
(a) The health, safety, and welfare of the child.
(b) Any history of abuse by one parent or any other person seeking
custody against any of the following:
(1) Any child to whom he or she is related by blood or affinity or
with whom he or she has had a caretaking relationship, no matter how
(2) The other parent.
(3) A parent, current spouse, or cohabitant, of the parent or
person seeking custody, or a person with whom the parent or person
seeking custody has a dating or engagement relationship.
As a prerequisite to the consideration of allegations of abuse,
the court may require substantial independent corroboration,
including, but not limited to, written reports by law enforcement
agencies, child protective services or other social welfare agencies,
courts, medical facilities, or other public agencies or private
nonprofit organizations providing services to victims of sexual
assault or domestic violence. As used in this subdivision, "abuse
against a child" means "child abuse" as defined in Section 11165.6 of
the Penal Code and abuse against any of the other persons described
in paragraph (2) or (3) means "abuse" as defined in Section 6203 of
(c) The nature and amount of contact with both parents, except as
provided in Section 3046.
(d) The habitual or continual illegal use of controlled substances
or habitual or continual abuse of alcohol by either parent. Before
considering these allegations, the court may first require
independent corroboration, including, but not limited to, written
reports from law enforcement agencies, courts, probation departments,
social welfare agencies, medical facilities, rehabilitation
facilities, or other public agencies or nonprofit organizations
providing drug and alcohol abuse services. As used in this
subdivision, "controlled substances" has the same meaning as defined
in the California Uniform Controlled Substances Act, Division 10
(commencing with Section 11000) of the Health and Safety Code.
(e) (1) Where allegations about a parent pursuant to subdivision
(b) or (d) have been brought to the attention of the court in the
current proceeding, and the court makes an order for sole or joint
custody to that parent, the court shall state its reasons in writing
or on the record. In these circumstances, the court shall ensure that
any order regarding custody or < parenting time > is specific as to time,
day, place, and manner of transfer of the child as set forth in
subdivision (b) of Section 6323.
(2) The provisions of this subdivision shall not apply if the
parties stipulate in writing or on the record regarding custody or
< parenting time >.