Note:  "Push models" suggest that some factor(s) pushed societies to become more complex; "pull models" suggest that some factor(s) permitted societies to become more complex.
 

Environmental / Ecological Explanations

something about the environment leads to complexity

may be a harsh environment, which challenges humans to adapt more complexity (push model)

may be a rich environment, which allows for food surpluses and social classes (pull model)

may be a diverse environment, which requires more interaction to get scattered resources (push model)
 

Population Increase Explanations

population growth may lead to more complicated social relationships that require an institutionalized means of guiding behavior (push model)

or, population increase may give incentive to develop more technologies and social organizations to deal with more people (pull model)
 

Technological Explanations

technology includes tools (ceramics, metallurgy, plows, bow and arrow, etc.) or writing

either inventions led to more efficient use of the environment and food surpluses, which allowed development of complexity (pull model)

or, the need to control surpluses and other materials led to development of technological methods of controlling trade, distribution and craft specialization (push model)

 
Irrigation  Explanations  (Julian Steward, Karl Wittfogel)

the need to increase food supplies led to the development of irrigation systems

irrigation systems required greater complexity to organize labor, to feed workers,  to plan and build  the canal system, to maintain canals, and to distribute water (push model)

 
Exchange Explanations

scattered resources in an area may led to a class of traders, demand for goods leads to craft specialists

food surpluses are required to support traders and craft persons, a means of transporting goods is required, a method of standardizing goods and services is needed, and an institutionalized means of controlling resource access and redistribution (pull model)

 
Environmental Circumscription  / Warfare    (Robert Carniero)

states arose in areas where the expansion of a growing population was restricted such that conflicts between groups over resources developed

conflict s among groups in restricted regions led to warfare and control of the losing group by the winning group

when this power becomes greater and is institutionalized, state-level societies result (pull model)
 

Religion  Explanations

early religious figures in a society functioned to insure fertility of the crops and herd and to control agricultural cycles like planting and harvesting, both through observation of the stars and ritual sacrifices

these roles led to control of food surpluses and redistribution of goods by the religious  leaders, which elevated their status and power in society

as power was gained and institutionalized and expanded beyond subsistence pursuits,  complex society (often a theocracy) was formed (pull model)
 

Class Conflict

disputes between social groups led to the need to develop an institutionalized means of resolving disputes

this led to many characteristics associated with states like strong, centralized governments, force as a means of social control and codified law

complex society developed as a problem solver; it developed to maintain order (pull model)
 

REFER TO THE FAGAN TEXT FOR MORE DETAILS