California Code, Labor Code - LAB § 4663

(a) Apportionment of permanent disability shall be based on causation.

(b) A physician who prepares a report addressing the issue of permanent disability due to a claimed industrial injury shall address in that report the issue of causation of the permanent disability.

(c) In order for a physician's report to be considered complete on the issue of permanent disability, the report must include an apportionment determination. A physician shall make an apportionment determination by finding what approximate percentage of the permanent disability was caused by the direct result of injury arising out of and occurring in the course of employment and what approximate percentage of the permanent disability was caused by other factors both before and subsequent to the industrial injury, including prior industrial injuries. If the physician is unable to include an apportionment determination in his or her report, the physician shall state the specific reasons why the physician could not make a determination of the effect of that prior condition on the permanent disability arising from the injury. The physician shall then consult with other physicians or refer the employee to another physician from whom the employee is authorized to seek treatment or evaluation in accordance with this division in order to make the final determination.

(d) An employee who claims an industrial injury shall, upon request, disclose all previous permanent disabilities or physical impairments.

(e) Subdivisions (a), (b), and (c) do not apply to injuries or illnesses covered under Sections 3212, 3212.1, 3212.2, 3212.3, 3212.4, 3212.5, 3212.6, 3212.7, 3212.8, 3212.85, 3212.9, 3212.10, 3212.11, 3212.12, 3213, and 3213.2.