Intended as a heartwarming tonic, "Why Would I Lie?" turns out to be indigestible sentimental ipecac. Failing to hit the spot they had in mind, screenwriter Peter Stone and director Larry Peerce would have us swallow the misconception that nothing is so endearing as a feckless young hero.

Their specimen is called Cletus Hayworth impersonated with charmless smirky gusto by Treat Williams.

A ne'er-do-well who supposedly grows up in the course of playing guardian, surrogate daddy and mommy-hunter for a foster child, Cletus wears out his welcome pronto by being introduced as a tiresome put-on artist, over-age flower child, whimsical simpleton and habitual liar. These attributes are endorsed in scenes with his analyst (Severn Darden), his older siblings (Anne Byrne, Nicolas Coster and Sonny Davis) and a welfare supervisor (Valerie Curtin) who interviews him for a job.

If they were steady moviegoers, they might be inclined to associate Cletus' peculiarities with too many viewings of "Harold and Maude" or "morgan!" His behavior is not so mucheccentric as derivative.

Spurning his share of a family inheritance, Gletus simultaneously frustrates his siblings and demonstrates his indifference to material gain. He is meant to appear more ingratiating yet by telling whoppers during the job interview and then betraying the trust of the woman who hires him, evidently because she's generous enough to give him the benefit of the doubt.

Valerie Curtin lends some incongruous human dignity to the material by projecting a touchingly susceptible complex of emotions in her secnes with Williams. She's suspicious, curious, disapproving, sympathetic and sexually vulnerable. Curtin seems far too responsive and genuine for this crummy context. Neither the hero nor the material really deserves such finely calibrated character comedy.

Soon after being hired as a case worker, Cletus exploits his position to perpetrate a hoax. He removes a little boy (gabriel Swan, an inexpressive look-alike of Ricky Schroder, the blond heart-tugger of "The Champ") from a perfectly devoted foster mother so that he can play foster father: he even misrepresents himself to the boy as his real father. At the same time, irrepressible Cletus pursues an unauthorized search for the child's natural mother, who put her infant up for adoption when she was sent to the slammer. This adventure brings him to a halfway house where the official love interest emerges in the form of Lisa Eichhorn, one of the counselors.

There's no doubt that this delightful duo is meant for each other. Cletus eavesdrops as the counselor, named Kay, presides over a group-therapy session where a teen-age girl provides a sizzling description of a grope with her boyfriend. When it's over, Cletus introduces himself and tries to turn on the charm. He needn't have bothered: "I don't know about you," Kay confesses, "but listening to that girl's story really made me horny."

Having reached an instant sexual accommodation, the relationship peaks prematurely, but the filmmakers stubbornly insist that Cletus and Kay are exemplary numbskulls and that the welfare and happiness of the deceived little boy depends on securing their permanent services as dad and mom. The plot is thickened with a shocking (sort of) revelation -- that Kay has been romantically entangled with the halfway house's director, a possessive lesbian played by Susan Heldfond and then an outrageous coincidence, but there's no averting an inanely rosy conclusion in the wake of the belated storm clouds.

A pity too, because Heldfond is an emotionally charged presence, flinty and demanding. Both Williams and Eichhorn, mostly a passive blur despite her character's initial dirty flirtiness, look seriously overmatched against Heldfond. Like Curtin in the earlier stages of the film, she injects unforseen complications into a script that remains defiantly trite.

The recent tendency to glorify the paternal instinct is fast becoming a self-congratulator joke induled by all too presumptuous (and maybe insecure?) male filmmakers. If Cletus can be considered a model of masculinity worth emulating, the trend has obviously hit the skids.