In virtually every way, including the size of their family, John and Paula Lawrence are quintessentially middle class. They live in a white brick Colonial in the leafy upper Northwest section of Washington. He is a commercial real estate lender. She is an audiologist. Their daughter, Candice, 12, attends private school, plays violin in a citywide youth orchestra and studies origami. Their son, John Paul, 6, has yet to develop the patience needed for such pursuits, but his parents are confident that in time he will.

The Lawrences, who devote a lot of time and energy to their children, are also adamant that they will have only two. In their decision to limit the size of their family, the Lawrences are an example of a little-noticed but striking trend in the nation’s black population: a steep decline in the birth rate for married African-American couples. Married black women gave birth to 357,262 babies in 1970. But by 1996, the last year for which complete figures were available, that figure had dropped to 179,568, a decline of nearly 50 percent, nearly twice the drop in the birth rate among married white women.

Most of the reasons for the sharp drop in child-bearing among black women are universal: higher family incomes, more education, movement away from parents and relatives who can provide support and child care. As more black families move up the economic ladder, they are following the well-worn path of other ethnic groups and reducing the size of their families. But some who have studied the phenomenon also say the slowdown in the birth rate among two-parent black families is caused by some forces not found in the larger white population. Black couples often want to maximize the benefits they can give to their children so that the offspring will have a better chance to compete in a society the parents feel is racist, these expert say.

”I’ve talked to numerous African-American parents who really are very serious about making sure their children are having rich, rewarding experiences so that they start life with an advantage and not disadvantage,” said Dr. William Turner, a professor of family studies at the University of Kentucky. The sharp drop in the birth rate for black married couples has resulted in something of a statistical conundrum. It has contributed to what has been described as a ”crisis” of illegitimacy in the black population, where nearly 70 percent of babies are born to single mothers. The number of single black women bearing children has increased steeply, rising to 415,213 in 1996, from 215,100 in 1970.

But statisticians and demographers point out that the startlingly high percentage of black children born outside of marriage is not merely the result of more single black women giving birth. The percentage of single black women giving birth has been declining since 1989, and reached a 40-year low in 1996. Instead, the high proportion of black babies being born out of wedlock is now mainly a function of its statistical comparison to the steep drop in the number of black children being born to married black women.

Experts say the trend means that married black couples, whose median household income is rapidly approaching parity with their white counterparts, are able to give their children more of the advantages of enhanced economic status, like better housing, better schools, more material goods, more opportunities for travel and cultural enrichment.

”I don’t think either one of us ever thought in terms of having three or four kids,” Mr. Lawrence said. ”I didn’t have any fast or hard ideas not to. I don’t think Paula had any fast and hard ideas not to. But I don’t think we ever really contemplated more than two.”

But experts warn that because incomes for single black mothers remain stagnant and the rates of their having children have not slowed as rapidly, the sharp decline in the percentage of married black families having children could widen the class divisions among blacks.

”I do think that the potential for these rifts to be exacerbated is terrific,” Dr. Turner said. ”Unless there is some constructive, meaningful dialogue and some organizations that are strong in the black community get involved and deal with this in a meaningful way, I think you will see those rifts grow even more and more and more.”