Ever wonder why, in a nation of such diversity, all 16-year-old girls from Englewood, N.J., to Inglewood, Calif., wear their hair and makeup exactly the same?

Because every season the National Hairdressers and Cosmetologists Association decides which trends are going to be popular -- not just for girls but for everyone, male and female. At the NHCA's "American Images '86" convention at the Washington Hilton through tomorrow, 6,000 hairdressers and cosmetologists are learning the new fall/winter 'dos and don'ts.

"We've designed the statements for the U.S.," stated Pauline McCloud, a member of the NHCA's Trend Release Design Team, which studied fashion statements and came up with complementary hair statements: the Celebrity, the Cosmopolitan and the Classic for women, and the Entertainer and the Croupier for men. For women, "the statement is elegance, it's softness, it's femininity, it's sophistication, it's convertible. Convertible is a very key element in this statement."

"The clothes make a very definite statement of being geometric," stated Jim Lee, another team member. "Each one of the hair designs makes a statement of its own. Whatever fashion statement you make, they can be adapted to that statement."

"The nail statement is snakeskin covers," stated Lynn James, chairman of the association's Nail Technicians-America division. "They're in seven different colors. They are real snakeskins, which add not only ornamentation but strength to the nail. I mean, it's leather. The other statement is pearl nail jewelry. You pierce the nail extension with a little drill and the pearl is on a post like an earring. And that's what the statement is."

Saturday's black-tie opening banquet (which drew as many red and lavender ties as black) featured "Knots Landing's" Lisa Hartman, named women's Style Maker of the Year, and her hairdresser-to-the-stars Jose Ebert. Hartman's golden locks were outshone by her clinging, pearl-encrusted gown. Ebert wore a lime green suit, a cowboy hat that looked more like a bran muffin, and a loosely braided ponytail.

Don Johnson, men's Style Maker of the Year, was more than fashionably late -- he didn't show up. But Jan Miner did. Miner, who plays Madge, the matronly manicurist in the Palmolive commercials, soaked in the glory as she accepted a special recognition award from the nails division. "This is like getting an Oscar to me," she said. "It really is just as exciting."

Yesterday's trend premiere introduced hairdressers to the Trend Release Design Team's creations (trends are made, not found) in a fashion show format in the ballroom. While a Madonna-like dancer gyrated to "Material Girl" and male and female models topped with the new "releases" strutted and posed, Myrtle Taylor of Vanity Beauty and Gift Shop in Aberdeen, S.D., wondered if her clients would accept change gracefully. "You bring in some of the new lines whether they know it or not," she said, "and they feel new."

Dwight Miller prepared for his show of futuristic hairstyles by covering Brooke Baldwin, 15, of Annandale with a bright orange wig and cutting it until it looked like a rice paddy on Moon Base Alpha.

Workshop participants cut into specialized topics, from emotions (this is a people business) to new cosmetic techniques. June A. Olson asked her students to "do some sharing" in her "Focus -- Channeling Your Energies" workshop. "I think it's very important that we remember how much 'visual' means to us but how little it has to do with what's inside us," she said.

Dawn Marie's "AromaTherapy" workshop introduced the art and science of making oils for skin treatment from such vegetable matter as basil, hyssop, juniper and sandalwood. But you can't just squeeze some bergamots and wash your face with the juice. "The essence is the life force of the plant, the purest, most therapeutic part of the plant," Marie said.

Beauty can be political. Skin care experts are lobbying for separate licensing procedures for skin and hair care establishments. Said Marina DeHaydu, manager of Christine Valmy Skin Care Salons in New York City, "I don't know how to cut hair, I'll never know how to cut hair, I don't want to know how to cut hair. But that doesn't mean I don't know how to squeeze a blackhead or a pimple."

Beauty can also be gross. Christine Valmy said the latest in skin care is facial masks made with 9- to 11-day-old living chicken embryos.

Beauty can also be a force for social change. "My dad was a truck driver," recalled Earl Roach, an association bigwig, "and if you didn't smell like a locker room towel you weren't a man."

In the exhibition hall, manufacturers hawked everything from familiar implements, such as Tamsco's 15 kinds of scissors and six kinds of tweezers, to newfangled contraptions such as Networked Picture Systems' $9,700 computer imaging system, which lets beauticians fiddle with hair and makeup on true-to-life pictures of their clients.

Do beauticians ever get gossip from their clients that's too cruel to be cool? "Yes, and I wouldn't want to repeat it," said Margaret Krokker, owner of Margaret Krokker's Beauty Salon in Dearborn, Mich. "They can find another hairdresser, you know."