Man, according to many psychologists, has a primordial itch to stake out his turf. He has claimed caves, built castles and created nations.

Suburban man, according to this theory, builds fences.

In the Washington suburbs and suburban boom-towns across the country, he is building so many fences that the fence industry has mushroomed into a nearly $4 billion-a-year business.

The Washington-based Long Fence Company decided 10 years ago to cash in on the suburban fence. Since then it's become the largest fence retailer on the East Coast, increasing its total sales of wooden "privacy" fences 50 times. This year alone Long Fence has sold 2 million board feet of fence wood.

"Given the level of isolation and paranoid distrust among suburban neighbors, fence building makes a lot of sense," said Robert C. Weigl, a social psychologist who counsels unhappy suburbanites in Northern Virginia.

There are, however, less lofty explanations for the fencing boom than those derived from the nature of man or those dependent on modes of alienation. Interviews with suburban man and suburban woman show that fences go up beacuse of:

Indiscriminate dogs, marauding children, fat neighbors in bathing suits, odoriferous mulch piles, rusting washing machines, garbage trucks, snoopy passers-by and clotheslines hung with underwear.

Fence owners also said they to keep their own problems -- their indiscriminate dogs, their marauding children, etc. -- from getting out and generating lawsuits.

Fence-building has grown to the point that even local politicians have noticed it.

Out in Fairfax County, the fastest growing suburban jurisdiction in the Washington area, Supervisor Marie B. Travesky (R-Springfield) said the fences are changing the nature of the suburbs.

"Used to be you'd go into a neighborhood and see nothing but green rolling lawns and trees. Now with the dogs and the kids and the search for privacy, people are doing things to set off their little corner of the world," Travesky said.

In his famous metaphysical musing on the meaning of fences, poet Robert Frost wrote that before he would build a fence he'd "ask to know what I was walling in or walling out."

If the great Yankee poet had lived in the Waldorf, Md. area 10 years ago, when the fence boom was getting started, he would have known better than to ask such a question.

According to Ken Werts, who's been erecting fences in Maryland for 15 years, this what happened:

"A guy saw his neighbor's horse leaning over his 42-inch fence, eating his flowers. The guy then when out to his back yard and whacked that horse with a switch. The horse backed off.

"The guy then went back inside his house and heard the doorbell ring. When he went to the door, his nextdoor neighbor [the one who owned the recently whacked horse] let the guy have it right between the eyes with a baseball bat."

Werts said he was called to the assaulted man's house to give advice on the cost of a 6-foot-high, horse-proof fence.

"The guy was all black and blue around the face. He was astounded that his neighbor would act that way," Werts said. The 6-foot fence went up immediately.

In examining reasons why suburban man, who is not afraid of his neighbor's baseball bat, appears to have a thirst for fences, the new field of environmental psychology has come up with tentative hypothesises.

Dr. Irwin Altman, dean of the College of Social and Behavioral Science at the University of Utah, says that fences (which cost an average of $1,500 when made of wood and built on an one-acre lot) may serve as "a symbolic barrier."

"In the United States we have had so much space for so long that we didn't need fences. But now with all the crowded development town houses, homes built on smaller and smaller lots, fences serve the purpose of smoothing out the interaction among people. They know where they can go and where they can't," Altman says.

Altman adds, of course, that his thoughts about fence building in the suburbs are "highly speculative" and that the field is lacking in solid research.

One sociologist at the University of Massachusetts recently began some solid research. He built a fence around his house and now thinks about why he did it.

"When your own property is merged with the street and the public domain, where is something unsatisfactory about it somehow, said Dr. Paul Hollander, who lives in Northhampton, Mass.

He says his ostensible reason for erecting the fence was to "keep out the dogs of the neighborhood and what they leave behind." But, the sociologist, concedes, the reasons for his fence run deeper than dogs.

"I would have built the fence even without the dogs. It is symbolic. It delineates my little realm. One it was there, I felt more free to do whatever I want to do with my property," Hollander said.

Around Washington, developers say the general rule is that the closer the houses are built, the higher, the fences.

At inverness North, a 124-unit town house development in Potomac, every town house has fencing that stands 6 feet, 6 inches high and is made with wooden slats that have sharp points at the top. From a distance, the development has the appearance of a maze for extremely large rats.

"When you live in a town house they make you feel closer to your neighbor than you ever imagined," said Patti Dickenson, a librarian who moved into the town house complex this summer.

Each town house owner has the option of adding a back fence to the side fences that come with the purchase of the town houses. By general consensus among owners, the back-fence options turns their 20-by-26-foot back yards into "little boxes."

Yet, despite the claustrophobic effect, 100 of the 124 owners have chosen the back fence. Most owners agree that they never meet a neighbor by hanging out in the back yard.

Katherine DeWitt, who had the back fence installed in her yard after suffering the stares of neighborhood kids while barbecuing last year, admits it would be nice if the fence allowed someone to stroll by the back yard and say "Hi."

But fenced-in privacy and back yard hellos don't mix, owners at Inverness North say. For them, privacy has the higher value.