In the Mesa Verde area of the ancient North American Southwest, living patterns changed in the thirteenth century, with large numbers of people moving into large communal dwellings called pueblos, often constructed at the edges of canyons, especially on the sides of cliffs. Abandoning small extended-family households to move into these large pueblos with dozens if not hundreds of other people was probably traumatic. Few of the cultural traditions and rules that today allow us to deal with dense populations existed for these people accustomed to household autonomy and the ability to move around the landscape almost at will. And besides the awkwardness of having to share walls with neighbors, living in aggregated pueblos introduced other problems. For people in cliff dwellings, hauling water, wood, and food to their homes was a major chore. The stress on local resources, especially in the firewood needed for daily cooking and warmth, was particularly intense, and conditions in aggregated pueblos were not very hygienic.

Given all the disadvantages of living in aggregated towns, why did people in the thirteenth century move into these closely packed quarters? For transitions of such suddenness, archaeologists consider either pull factors (benefits that drew families together) or push factors (some external threat or crisis that forced people to aggregate). In this case, push explanations dominate.

Population growth is considered a particularly influential push. After several generations of population growth, people packed the landscape in densities so high that communal pueblos may have been a necessary outcome. Around Sand Canyon, for example, populations grew from 5 -12 people per square kilometer in the tenth century to as many as 30 - 50 by the 1200s. As densities increased, domestic architecture became larger, culminating in crowded pueblos. Some scholars expand on this idea by emphasizing a corresponding need for arable land to feed growing numbers of people: construction of small dams, reservoirs, terraces, and field houses indicates that farmers were intensifying their efforts during the 1200s. Competition for good farmland may also have prompted people to bond together to assert rights over the best fields.

Another important push was the onset of the Little Ice Age, a climatic phenomenon that led to cooler temperatures in the Northern Hemisphere. Although the height of the Little Ice Age was still around the corner, some evidence suggests that temperatures were falling during the thirteenth century. The environmental changes associated with this transition are not fully understood, but people living closest to the San Juan Mountains, to the northeast of Mesa Verde, were affected first. Growing food at these elevations is always difficult because of the short growing season. As the Little Ice Age progressed, farmers probably moved their fields to lower elevations, infringing on the lands of other farmers and pushing people together, thus contributing to the aggregations. Archaeologists identify a corresponding shift in populations toward the south and west toward Mesa Verde and away from higher elevations.

In the face of all these pushes, people in the Mesa Verde area had yet another reason to move into communal villages: the need for greater cooperation. Sharing and cooperation were almost certainly part of early Puebloan life, even for people living in largely independent single-household residences scattered across the landscape. Archaeologists find that even the most isolated residences during the eleventh and twelfth centuries obtained some pottery, and probably food, from some distance away, while major ceremonial events were opportunities for sharing food and crafts. Scholars believe that this cooperation allowed people to contend with a patchy environment in which precipitation and other resources varied across the landscape: if you produce a lot of food one year, you might trade it for pottery made by a distant ally who is having difficulty with crops—and the next year, the flow of goods might go in the opposite direction. But all of this appears to have changed thirteenth century. Although the climate remained as unpredictable as ever between one year and the next, it became much less locally diverse. In a bad year for farming, everyone was equally affected. No longer was it helpful to share widely. Instead, the most sensible thing would be for neighbors to combine efforts to produce as much food as possible, and thus aggregated towns were a sensible arrangement.

在古代北美西南部的梅萨维德 地区，生活模式在十三世纪发生了变化，大量人群移居大型公社居住地，这种地方被称为普韦布洛，通常建造在峡谷边缘，尤其是在悬崖边。这些住户放弃小的扩展型家庭， 进入到没有上百也有几十人的大的普韦布洛地区可能会感到不舒服。 现在帮助我们处理密集人口的文化传统与规则几乎不存在于这些习惯了家 庭自治和有能力随意搬迁的人。 除了必须和邻居共用墙壁的尴尬外， 住在人口聚集的普韦布洛地区产生了其他的问题。对于那些住在悬崖上的人，拉水、木头和食物到家里是主要的家务。 本地资源的压力尤其是用于日常做饭和取暖的柴火特别紧张， 在人口聚集的普韦布洛的环境也不是很卫生。 考虑到住在人口密集地区的种种不利条件， 为什么人们会在十三世纪搬进这一如此密集的地区呢？对于这些突然的转变， 考古学家考虑到了拉力因素 （吸引家庭聚在一起的好处）和推力因素（迫使人们聚在一起的外部威胁或危机） 。在这种情况下，推力的解释更加占上风。 人口增长被认为是一个特别有影响力的推力。 在经历几代的人口增长之后， 人们使得这一地区的人口密度达到了如此高的程度以至于普韦布洛社区成为一个必需的结果。 比如在砂峡谷，十世纪时每平方公里 5 到 12 人，到十三世纪时增加到了每平方公里 30 到50 人。随着人口密度的增加，民房建筑变得更大，在拥挤的普韦布洛地区达到顶峰。一些学者通过强调养活越来越多的人口需要相应的耕地扩展了这一看法： 小型水坝、 水库、 梯田以及房屋的建设表明十三世纪的农民一直在加紧努力。 对于良田的竞争也会促使人们团结起来争取最好土地的权利。 另外一个推动力量是小冰河时代的到来， 这种气候现象导致了北半球气温的降低。 尽快尚未到达小冰河时代的巅峰时期， 一些证据仍表明十三世纪当时温度正在降低。 与这种转变相关的环境变化并未受到充分认识， 但是住在圣胡安山脉和普韦布洛地区的人们首先受到了影响。由于生长季节短，在这些海拔种植食物总是很困难。随着小冰河时代的到来， 农民们可能将他们的耕地迁到更低的海拔地区去， 侵入其他农民的土地并使人口聚集在一起， 因而促进了群居。 考古学家确认了向梅萨维德地区南部和西部并逃离更高海拔造成的人口变化。 在所有这些推动力面前， 梅萨维德地区的人们还有另外一个搬进公共村庄的理由： 对更多合作的需求。 分享和合作几乎可以肯定是早期普韦布洛人生活的一部分， 即使是那些分散在这一地区独立性比较强的单一家庭住宅也是这样。 考古学家发现在十一世纪和十二世纪期间，即使是最与世隔绝的居民也能从很远的地方获得陶器，可能还有物，而重大的宗教活动为这种分享食物和手工艺品提供了机会。 学者们相信这种合作让人们在这种拼凑的但不断变化的环境中能相互竞争：如果在某一年，你收获了很多粮食作物，你可能会与远方一个不太容易获得农作物的盟友交换陶器， 下一年货物可能会流向相反的方向。 但是所有这一切似乎已经改变了十三世纪。 尽管气候在一到两年间仍无法预测，但是在局部不会有太大变化。在收成差的年份里，每个人都会受影响。广泛共享已经不再有益了。取而代之，最明智的是一个地区的人们齐心协力生产尽可能多的食物，这样人口聚集的城市就是一个合理的安排。