THE END OF AFFLUENCE: The Causes and Consequences of America’s Economic Dilemma by Jeffrey Madrick (Random House: $22; 223 pp.) Former NBC economics reporter Jeff Madrick has lit upon a single statistic to explain the apparent economic decline in the United States over the last two decades--the fact that our rate of growth has fallen by a third since 1973, from the historical average of 3.4% to 2.3%. Some readers will no doubt assume that Madrick’s emphasis of this figure is a sign of television’s ability to oversimplify any issue, but in fact Madrick gives a cogent argument for its being the quintessential fact of modern economic life in this country. Considering the imprecision of many common fiscal figures, and the ease with which they can be massaged, one must take Madrick’s highlighting of the 2.3 statistic with a grain of salt, but by the same token it dovetails nicely with broader historical truths--the fact that the United States must now compete in a world marketplace, no longer enjoys the benefits of frontier-era monopoly, has been slow to change conventional business methods in the face of more nimble foreign competition. Madrick may talk of “the end of affluence,” but what he’s really saying--bottom line--is that if the United States hopes to regain world economic leadership it had better drop its starry-eyed optimism, refuse to coast on past accomplishments and roll up its sleeves for the long haul.