What 20th-century development most altered the human condition? There is no shortage of candidates: the automobile, antibiotics, the airplane, computers, contraceptives, radio and television, to name a few. But surely the largest advance in human well-being involves the explosion of freedom. In a century scarred by gulags, concentration camps and secret-police terror, freedom is now spreading to an expanding swath of humanity. It is not only growing but also changing--becoming more ambitious and ambiguous--in ways that might, perversely, spawn disappointment and disorder in the new century.

In 1900 this was unimaginable. "Freedom in the modern sense [then] existed only for the upper crust," says political sociologist Seymour Martin Lipset of George Mason University. There were exceptions--America certainly, but even its freedom was curtailed. In 1900 women could vote in only four western states. Not until the ratification of the 19th Amendment in 1920 could all women vote. In the South, a web of laws prevented black Americans from voting. It took the Voting Rights Act of 1965 to change that.

Elsewhere the picture was bleaker. In 1900 empires dotted the world. The British Empire contained roughly 400 million people, about a quarter of the world's population. Lesser empires were still enormous: the Austro-Hungarian, the Ottoman, the French and others. Human subjugation was the rule, not the exception.

Consider the situation now. In 1999 Freedom House--a watchdog group based in Washington--classified as "free" 88 of the world's 191 countries, with 2.4 billion people or about 40 percent of the total. These nations enjoyed free elections and traditional civil rights of speech, religion and assembly. Of course, there are shades of gray. In this twilight zone Freedom House placed 53 countries with 1.6 billion people, because either elections or civil liberties were compromised. Russia was "partially free"; China was "not free."

Still, the world's frame of reference has fundamentally altered. Even in societies where freedoms are abused, their absence usually becomes an issue. But freedom has not simply spread. It's also evolved, especially in the United States. The freedom that Americans expect as they enter the 21st century is not the same as the freedom they expected as they entered the 20th.

Traditional freedom historically meant liberation from oppression. But now freedom increasingly involves "self-realization." People need, it's argued, to be freed from whatever prevents them from becoming whoever they want to be. There's a drift toward "positive liberty" that emphasizes "the things that government ought to do for us," says sociologist Alan Wolfe of Boston College. This newer freedom blends into individual "rights" (for women, minorities, the disabled) and "entitlements" (for health care, education and income support) deemed essential for self-realization.

The broader freedom is not just American. In a new book, "Development as Freedom," the Nobel-Prize-winning economist Amartya Sen argues that "the expansion of freedom is both the primary end and . . . principal means of development" in poorer countries. But Sen's freedom eclipses the classic political and economic freedoms. It includes "social opportunities" (expanded education and health care) "transparency guarantees" (a lack of corruption) and more "entitlements" (to ensure basic decency and prevent "abject misery"). Indeed, it seems to include almost anything that might advance human well-being.

In some ways, freedom's explosion connects the century's two great constants: war and economic progress. Deaths in World War I and World War II are crudely reckoned at 10 million and as many as 60 million, respectively. But these vast tragedies ultimately paid some dividends for common people, because they doomed colonial empires. Also, the nature of the wars emphasized freedom. They were too destructive to be mere contests of nations. They had to be about ideals. The Cold War--an ideological conflict--conveyed the same message.

If war expanded freedom, prosperity embellished it. Since 1900 the world's population has roughly quadrupled, from almost 1.6 billion to 6 billion. Meanwhile, the global production of goods and services--from food and steel to air travel and health care--has risen 14 to 15 times, estimates economist Angus Maddison for the Organization for Economic Cooperation and Development in Paris. As nations grew wealthier, traditional freedom wasn't enough. People ascended what psychologist Abraham Maslow called the human hierarchy of needs--from food and shelter to self-esteem and spiritual needs, such as justice and beauty. People could not (it was said) be "free" without realizing these larger yearnings.

Freedom's fate in the next century is fragile, in part because the very notion is now so ill-defined. Classic freedom--coupling the opportunity for success with the danger of failure--hardly ensures personal fulfillment or social order. "On the one hand, you're told you're free," says Lipset. "But on the other, you're a potential loser. And if you lose, you don't feel free." The traditional freedoms of belief and lifestyle also require, if they are not to foster anarchy, tolerance and self-restraint.

But at least traditional freedom is universal. Everyone can, in theory, enjoy the freedoms of speech, religion, assembly and property. This sort of freedom promises the absence of coercion. By contrast, the new freedoms of individual "rights" and "entitlements" are increasingly exclusive, can involve social competition for benefits and may mean the subtle (or not so subtle) coercion of one group by another--all tending to weaken a sense of community. The "rights" of women, gays and the disabled cannot be directly enjoyed by men, straights or the nondisabled. Financing entitlements means taxes--a form of collective coercion--by which taxpayers subsidize beneficiaries.

Freedom, always a combustible concept, promises to become more so, because in a world of television and the Internet, ideas glide almost spontaneously across cultural and political boundaries. The eagerness of the West to export its ideals may increasingly collide with the willingness and capacity of others to abandon or modify their own. What we value, they may fear or mishandle. Freedom is a great blessing. But it has never been easy--and never will be.