Within the past year, scientists studying depression have found themselves confronted with what one researcher calls a "shocking" phenomenon: The inexplicable but significant increase in major depressions, their much earlier onset and the rise in suicides among those born since 1940.

Officially, it is known as "the cohort effect" -- found in a specific group, or cohort, of people determined in this case by year of birth. Among themselves, the scientists, at a loss to explain its cause, are calling it "agent blue."

Dr. Elliot S. Gershon of the clinical neurogenetics branch of the National Institute of Mental Health has found that not only has depressive illness in general increased among those born since 1940, but the most severe form, manic-depression, also has risen. This is a development he considers "ominous."

"This rapid change in rate of both clinical depression and manic-depression cannot be due to genetics. Genes don't change that fast. But it can be familial." Indeed, he said, "there is still a lot more illness in relatives of patients than in relatives of controls."

Gershon believes the increase is "either a genetic/environmental interaction, or a cultural event which is peculiar to families already vulnerable to mood disorders."

Because up to 70 percent of suicides in this country are among people suffering from a major mood disorder, Gershon says, "people who believe they can explain the increase in teen-age suicide, I think, are jumping to conclusions" if they are not taking the "cohort effect" into consideration.