[长难句突破]

This development - and its strong implications for US politics and economy in years ahead - has enthroned the South as America's most densely populated region for the first time in the history of the nation's head counting.

主体句式：This development has enthroned...

结构分析：这个句子的复杂成分在于两个破折号之间的补充说明内容。这个部分虽然以and开头，但是它与主语不是并列的关系，而是对于主语的补充说明。

[全文译文]

Emerging from the 1980 census is the picture of a nation developing more and more regional competition, as population growth in the Northeast and Midwest reaches a near standstill.

This development - and its strong implications for US politics and economy in years ahead - has enthroned the South as America's most densely populated region for the first time in the history of the nation's head counting.

Altogether, the US population rose in the 1970s by 23.2 million people - numerically the third largest growth ever recorded in a single decade. Even so, that gain adds up to only 11.4 percent, lowest in American annual records except for the Depression years.

Americans have been migrating south and west in larger number since World War II, and the pattern still prevails.

Three sun belt states - Florida, Texas and California - together had nearly 10 million more people in 1980 than a decade earlier. Among large cities, San Diego moved from 14th to 8th and San Antonio from 15th to 10th - with Cleveland and Washington.DC, dropping out of the top 10.

Not all that shift can be attributed to the movement out of the snow belt, census officials say, Nonstop waves of immigrants played a role, too - and so did bigger crops of babies as yesterday's "baby boom" generation reached its child bearing years.

Moreover, demographers see the continuing shift south and west as joined by a related but newer phenomenon: More and more, Americans apparently are looking not just for places with more jobs but with fewer people, too. Some instances-

 ● Regionally, the Rocky Mountain states reported the most rapid growth rate - 37.1 percent since 1970 in a vast area with only 5 percent of the US population.

●Among states, Nevada and Arizona grew fastest of all: 63.5 and 53.1 percent respectively. Except fro Florida and Texas, the top 10 in rate of growth is composed of Western states with 7.5 million people - about 9 per square mile.

The flight from overcrowdedness affects the migration from snow belt to more bearable climates.

Nowhere do 1980 census statistics dramatize more the American search for spacious living than in the Far West. There, California added 3.7 million to its population in the 1970s, more than any other state.

In that decade, however, large numbers also migrated from California, mostly to other parts of the West. Often they chose - and still are choosing - somewhat colder climates such as Oregon, Idaho and Alaska in order to escape smog, crime and other plagues of urbanization in the Golden State.

As a result, California's growth rate dropped during the 1970s, to 18.5 percent - little more than two thirds the 1960s' growth figure and considerably below that of other Western states.

1980年美国人口普查表明：随着东北部和中西部人口增长近乎停止，地区间的竞争越来越激烈了。

这一发展趋势以及它对今后几年在美国政治和经济的重大影响使南部在美国人口普查史上首次成为人口最密集的地区。

20世纪70年代，美国人口总共增长了2320万--从数字上看，这是有纪录以来10年期人口增长的第三高峰。即使如此，人口总数也只增加了11.4％，除了大萧条时期，这是美国最低的年度增长率。

自从第二次世界大战以来，美国人口一直大量向南部和西部迁移，而且至今这种趋势仍然盛行。

佛罗里达、得克萨斯和加利福尼亚这三个阳光充沛的州，1980年的人口比10年前增加了近1000万。在大城市排行榜上，圣地亚哥从第14位上升到第8位，圣安东尼奥从第15位升到第10位，而克利夫兰和华盛顿特区却被挤出了前10位。

人口普查官员说，并非所有这些人口迁移都是为了离开寒冷地带。不断涌入的移民潮，还有以前"生育高峰"期出生的那些孩子也已到了生育年龄，这些因素都在起作用。

此外，人口统计学家发现，向南部和西部的不断迁移还伴随着一种与此相关却又与以往不同的现象：越来越多的美国人显然不再仅仅寻找有更多就业机会的地方，他们也在寻找人口稀少的地方。请看以下例子：

从区域上看，洛基山脉附近各州上报了自1970年以来最高的人口增长率--37.1％，而以前这片广袤土地上的人口仅占美国总人口的5％。

从各州情况看，内华达和亚利桑那是增长率最高的两个州：其增长率分别为63.5％和53.1％。除了佛罗里达州和得克萨斯州外，处于人口增长率前10位的都是西部各州，共有750万人--每平方英里约9个人。

离开人口过度稠密区的做法影响了以前那种离开寒冷地带去气候宜人之地的趋势。

没有哪次人口调查比1980年美国人口普查更能突出显示美国人迁往最西部是为了找到更广阔的生存空间。正因为如此，加利福尼亚州在70年代人口增加了370万，比其他任何州都多。

70年代也有大量的人离开加利福尼亚，大多数去了西部其他各州。他们当时常常选择--现在依然这样选择--一些气候较冷的地区，如俄勒冈、爱达荷和阿拉斯加，为的是躲开"金州"加利福尼亚）的烟雾、犯罪和其他城市化进程中的问题。

结果，加利福尼亚的人口增长率在70年代降到了18.5％--略高于60年代增长率的2/3，但明显大大低于西部其他各州。