Opinion
REYNOSO, J.In 1980 the Legislature amended the Government Code section dealing with service-connected disability retirement of public employees.1 The new wording required that employment contribute substantially to an employee’s incapacity before the employee could qualify for *574such retirement. Did the amendment change the test for industrial causation, or merely clarify existing law? As we detail below, we conclude that the causation test was not changed.
Thomas Bowen applied for a service-connected disability retirement under section 31720. The Board of Retirement of the Los Angeles County Employees’ Retirement Association (Board) denied his application; instead, he was awarded a nonservice-connected pension.2 The trial court denied Bowen’s writ petition for a service-connected disability retirement and the Court of Appeal affirmed. We reverse and remand.
I.
Bowen worked for the County of Los Angeles as a stenographer from 1956 to 1975, and as an eligibility worker from 1975 to 1977. In May 1977, he broke his ankle in a nonservice-connected accident and never returned to work. Bowen testified that he decided not to return to work because he could not face the stress he experienced in performing his duties, particularly as an eligibility worker. He has been employed only sporadically since May 1977, having found any work situation too stressful.
In May 1978, a psychiatrist found that Bowen was temporarily disabled, and suffering from symptoms of anxiety, depression and alcohol abuse as a result of work stress as an eligibility worker. Bowen applied for a service-connected disability retirement in August 1978. Three doctors examined him between October 1978 and July 1979, and generally found that he had no permanent disability. In September 1979, the Board notified him that he was not disabled from his duties and denied his pension application.
Bowen suffered a heart attack in late July 1979, which apparently caused some permanent heart damage. Medical and psychiatric reports prepared during 1980-1981 indicate that he is totally disabled, attributing from 0 to 50 percent of his disability to industrial factors. At Bowen’s request, a hearing before a Board referee was held in November 1981. Based on the referee’s recommendation, the Board granted Bowen a nonservice-connected disability pension. The trial court denied Bowen’s petition for a peremptory writ of mandate for a service-connected pension and the Court of Appeal affirmed.
II.
Bowen initially applied for service-connected disability retirement in August 1978. Before his November 1981 hearing, however, the Legislature *575passed an amendment to section 31720 (the 1980 amendment) that became effective on January 1, 1981. Prior to the 1980 amendment, section 31720 provided in pertinent part: “Any member permanently incapacitated for the performance of duty shall be retired for disability regardless of age if, and only if: (a) His incapacity is a result of injury or disease arising out of and in the course of his employment . . . .” The 1980 amendment added the following clause to subdivision (a): “and such employment contributes substantially to such incapacity.” It also provided that this “substantial contribution” test “shall be applicable to all applicants for disability retirement on or after the effective date” of the amendment (Jan. 1, 1981).
Bowen contends that the 1980 amendment should not be applied retroactively to persons who applied before its effective date. He bases this first contention on two grounds: (1) rules of construction prevent retroactive application of a statute unless the Legislature expressly and clearly declares its intent that a statute operate retroactively (DiGenova v. State Board of Education (1962) 57 Cal.2d 167, 176 [18 Cal.Rptr. 369, 367 P.2d 865]; Flint v. Sacramento County Employees’ Retirement Assn. (1985) 164 Cal.App.3d 659, 663 [210 Cal.Rptr. 439]); and (2) any retroactive application of a stricter standard of causation under section 31720 would unconstitutionally impair his vested contractual right to a pension. (Allen v. City of Long Beach (1955) 45 Cal.2d 128, 131 [287 P.2d 765]; Abbott v. City of Los Angeles (1958) 50 Cal.2d 438, 453 [326 P.2d 484].)
Alternatively, in his second contention, Bowen asserts that the trial court and Court of Appeal misinterpreted the substantial contribution test of the 1980 amendment to section 31720. We need only address his second contention in order to resolve this case.3
In Heaton v. Marin County Employees Retirement Bd. (1976) 63 Cal.App.3d 421 [133 Cal.Rptr. 809], the court interpreted section 31720 in the context of an employee’s claim for a service-connected disability pension based on her mental disability. In that case, the Board appealed from a trial court’s writ of mandate directing it to grant the employee a service-connected pension. The Board argued that section 31720 required that the employment “be the sole or at least the substantial contributing cause of the disability before retirement disability may be awarded." (Id., at pp. 425-426.) The Heaton court rejected this argument, emphasizing that *576section 31720 required only that an incapacity be “‘a result of injury or disease arising out of and in the course of his employment,’ not the result thereof.” (Id., at p. 428, original italics.)
In a related argument, the Board contended that employment must be the sole cause of disability when the employee’s permanent incapacity results from mental, rather than physical disability. Otherwise, according to the Board, a psychiatrist would not be able to delimit causation of psychiatric disorders in cases of mental disability. Thus, even an “infinitesimal contribution to the disability might require full contribution.” (Id., at pp. 430-431.) The Heaton court also rejected this argument, noting that section 31722 explicitly provided for mental as well as physical disabilities. The court concluded that it had “no authority to rewrite the law to conform to [the Board’s] view of what it should be. [The Board’s] complaint is with the Legislature.” (Id., at p. 431.)
After the Heaton decision, some trial courts interpreted its holding to mean that even an infinitesimal or inconsequential work-related contribution to disability would suffice for a service-connected disability retirement. (Van Hook v. Board of Retirement (1983) 148 Cal.App.3d 714, 716, fn. 1 [196 Cal.Rptr. 186]; DePuy v. Board of Retirement (1978) 87 Cal.App.3d 392, 396 [150 Cal.Rptr. 791, 12 A.L.R.4th 1150].) In reaction to this interpretation of Heaton, many county governments advocated an amendment to section 31720, fearing that the courts’ liberal interpretation of industrial causation could threaten the viability of their retirement systems. (Assem. File Analysis, Sen. Bill No. 1076, June 5, 1980.) As originally introduced, the amendment allowed for a service-connected retirement only if the “permanent incapacity is the principal result of a substantial employment-caused injury or disease.” (Sen. Bill No. 1076 (1979-1980 Reg. Sess.) § 1, italics added.) A Senate analysis of the bill stated that the Heaton court ‘ ‘ determined that existing law allows an industrial disability retirement allowance to be granted even if the injury in question is not entirely service-connected .... This bill is an attempt to address that court decision.” (Sen. Com. on Pub. Employment & Retirement, May 14, 1979, italics added.)
Following a hearing by the Assembly Committee on Public Employees and Retirement, however, Senate Bill No. 1076 was amended to its present form, including the substantial contribution test for industrial causation. Significantly, the Assembly amendment deleted the requirement that a permanent incapacity be a principal result of an employment-caused injury from the bill’s final form. In the Assembly Third Reading, the analysis notes that the bill is a response to Heaton, supra, 63 Cal.App.3d 421, in which the court stated that “a member is entitled to a service-connected disability if (1) he or she is permanently unable to perform his or her job, and (2) *577any part of the disability is job-connected.” (Assem. Third Reading file analysis, June 5, 1980, original italics.) The analysis also stated that the amendments to Senate Bill No. 1076 “reflect a compromise between supporters of the bill and representatives from labor who strenuously opposed the bill as it was introduced.” (Ibid.) After the Senate concurred in the amended bill, the Governor signed it into law.
In this case, the Board contends that we should interpret the substantial contribution test of section 31720 to mean more than 50 percent industrial causation. We cannot discern such a purpose in the legislative history of the 1980 amendment. As the court in Gatewood v. Board of Retirement(1985) 175 Cal.App.3d 311, 318 [220 Cal.Rptr. 724], concluded: “[T]he Legislature intended to disavow the implications of Heaton but did not go so far as requiring the disability [to] be the [principal] result of an employment caused injury or illness. In other words, ‘contributes substantially’ means more than ‘any’ and less than ‘[principal].’ Beyond this rough bracketing, however, the Legislature provided no clear guide to interpret ‘contributes substantially. ’ ” (See also Lundak v. Board of Retirement (1983) 142 Cal.App.3d 1040, 1045 [191 Cal.Rptr. 446] [Legislature did not promulgate any quantitative guidelines for the word “substantially”].)
In our examination of the 1980 amendment to section 31720, we are guided by the underlying purpose of pension legislation, which “recognize^) a public obligation to . . . employees who become incapacitated by age or long service in public employment and its accompanying physical disabilities by making provision for retirement compensation and death benefit as additional elements of compensation for future services . . . .” (§ 31451.) In accordance with this stated purpose, “pension legislation must be liberally construed and applied to the end that the beneficent results of such legislation may be achieved. Pension provisions in our law are founded upon sound public policy and with the objects of protecting, in a proper case, the pensioner and his dependents against economic insecurity. In order to confer the benefits intended, such legislation should be applied fairly and broadly.” (Cordell v. City of Los Angeles (1944) 67 Cal.App.2d 257, 266 [154 P.2d 31]; accord, Gorman v. Cranston (1966) 64 Cal.2d 441, 444 [50 Cal.Rptr. 533, 413 P.2d 133]; Eichelberger v. City of Berkeley (1956) 46 Cal.2d 182, 188 [293 P.2d 1].)
In the 1980 amendment to section 31720, the Legislature intended to disapprove not the entire body of case law construing that section, but only the “infinitesimal contribution” language in Heaton. (Gatewood v. Board of Retirement, supra, 175 Cal.App.3d at p. 319.) Therefore, we may rely on prior case law to define the appropriate test for industrial causation *578under section 31720.4 For example, in DePuy v. Board of Retirement, supra, 87 Cal.App.3d 392, 398-399, the court stated that an “infinitesimal” or “inconsequential” connection between employment and disability would be insufficient for a service-connected disability retirement. Instead, the court concluded that “while the causal connection between the [job] stress and the disability may be a small part of the causal factors, it must nevertheless be real and measurable. There must be substantial evidence of some connection between the disability and the job.” (Id., at p. 399, italics added; cf., Gelman v. Board of Retirement (1978) 85 Cal.App.3d 92, 97 [149 Cal.Rptr. 225] [requiring a “material and traceable” connection between employment and disability].)
This formulation of the substantial contribution test, requiring substantial evidence of a “real and measurable” connection between the disability and employment, would not disturb the Legislature’s intent to reject the Heaton decision (supra, 63 Cal.App.3d 421). The substantial contribution test “would not include any contribution of employment to disability, no matter how small and remote.” (Lundak v. Board of Retirement, supra, 142 Cal.App.3d at p. 1046, original italics.) “Indeed, once the Heaton implications are checked, there is no significant difference between the pre- and postamended section 31720 tests for disability.” (Gatewood v. Board of *579Retirement, supra, 175 Cal.App.3d at p. 319, original italics.) In addition, this definition of substantial contribution also comports with the principle that pension legislation be applied fairly and broadly.
In this case, both the trial court and Court of Appeal rejected the DePuy test for a service-connected disability retirement, reasoning that DePuy was decided before the 1980 amendment to section 31720. However, we conclude that the 1980 amendment to section 31720 rejected Heaton only, and did not change the test for service-connected disability. Accordingly, we reverse the Court of Appeal, and direct that court to remand this case to the trial court so that it may apply the proper test of causation for a service-connected disability retirement when it exercises its independent judgment in evaluating the evidence produced before the Board. (Strumsky v. San Diego County Employees Retirement Assn. (1974) 11 Cal.3d 28, 34 [112 Cal.Rptr. 805, 520 P.2d 29].)
Bird, C. J., Mosk, J., Broussard, J., and Grodin, J., concurred.
In this opinion, all statutory references are to the Government Code.
The amount of an employee’s pension varies depending on whether his disability is service connected or nonservice connected. (See §§ 31727, 31727.4, and 31727.7.)
Because the 1980 amendment does not change the test for service-connected disability retirement, there is no need to reach Bowen’s arguments regarding the amendment’s retroactive application. “An exception to the general rule that statutes are not construed to apply retroactively arises when the legislation merely clarifies existing law. (Martin v. California Mut. B. & L. Assn. (1941) 18 Cal.2d 478, 484 [116 P.2d 71].)” (Balen v. Peralta Junior College Dist. (1974) 11 Cal.3d 821, 828, fn. 8 [114 Cal.Rptr. 589, 523 P.2d 629].)
Bowen urges that we follow the interpretation of “contributes substantially” suggested by the court in Lundak v. Board of Retirement, supra, 142 Cal.App.3d 1040. After noting that the Legislature did not promulgate any quantitative guidelines concerning the meaning of “substantially” in section 31720, the Lundak court turned to the Restatement Second of Torts for assistance in defining that word. The court stated: “One of the requirements for ‘legal cause’ set forth in section 431 of the Restatement Second of Torts is that negligent conduct be a ‘substantial factor’ in bringing about harm. Comment a to section 431 distinguishes substantial cause from cause in a philosophical sense in the context of negligent conduct: ‘The word “substantial” is used to denote the fact that the defendant’s conduct has such an effect in producing the harm as to lead reasonable men to regard it as a cause, using that word in the popular sense, in which there always lurks the idea of responsibility, rather than in the so-called “philosophic sense,” which includes every one of the great number of events without which any happening would not have occurred. Each of these events is a cause in the so-called “philosophic sense,” yet the effect of many of them is so insignificant that no ordinary mind would think of them as causes.’” (Id., at pp. 1045-1046.)
It could be potentially confusing to use tort definitions of causation to define the phrase “contributes substantially” in section 31720. Generally, courts have found that the County Employees Retirement Act of 1937 (here at issue) and the Workers’ Compensation Act “are related in subject matter and harmonious in purpose.” (Kuntz v. Kern County Employees’ Retirement Assn. (1976) 64 Cal.App.3d 414, 421 [134 Cal.Rptr. 501]; accord, Minor v. Sonoma County Employees Retirement Bd. (1975) 53 Cal.App.3d 540, 544 [126 Cal.Rptr. 16].) In fact, courts have looked to workers’ compensation law precedent for guidance in contending with similar issues in pension law. (Buckley v. Roche (1931) 214 Cal. 241, 245 [4 P.2d 929]; Kuntz v. Kern County Employees’ Retirement Assn., supra, 64 Cal.App.3d at p. 421.) In Maher v. Workers’ Comp. Appeals Bd. (1983) 33 Cal.3d 729, 734, footnote 3 [190 Cal.Rptr. 904, 661 P.2d 1058], we recently concluded that a tort definition of proximate cause is more restrictive than that required in workers’ compensation cases. A similar approach seems warranted for pension law cases. Therefore, we choose not to follow the definition of “substantially” suggested by the Lundak court.