Monday, August 5, 2013

Human Cultural Evolution: The Boserup Theory

By Lynden S. Williams


This is the second of a three part series in which we will examine the ‘demand-side’ or Boserup Theory of human cultural evolution.  In Part One I laid out the ‘supply-side’ or Malthusian Theory.  I strongly recommend that you read Part One first.  In Part Three I will argue in favor of the Boserup Theory and discuss some of the implications of human cultural evolution from that perspective.

The Boserup Theory

The Boserup Theory[1] can be stated in the form of three postulates and a logical deduction.   Postulate One:  An increase in production on a fixed amount of land will require more work at a lower wage, unless diminishing returns to labor are offset by more productive technology (Law of Diminishing Returns).  Postulate Two: People will resist changing their production methods when those changes increase work and reduce wages, unless they are forced to do so by an increase in demand (Law of Least Effort[2]).  Postulate Three: The necessity of working longer at a lower wage, along with positive aspects of higher population density, will tend to stimulate innovation and the adoption of more productive technology.  Logical Deduction: An increase in demand (which for most of human history resulted from population growth) is the cause for increased production, and is a necessary component of increasing production per capita (economic development). 
Boserup did not present her theory in the form of the three postulates and the logical deduction used here.  I have used that method to conform to the presentation of the Malthusian Theory presented previously.  Of course, Boserup was not the first to recognize the concept that ‘necessity is the mother of invention.’   As is true of Malthus and his theory, Boserup took bits of common knowledge and built them into a theoretical model that laid out the implications for human cultural evolution.  I have found a short restatement of her theory by Brian Spooner an excellent summary more easily grasped by students.[3]

The Historical Sequence of Land Use Intensification

The first postulate is simply a statement of the Law of Diminishing Returns to Labor, and therefore, an indisputable fact.  Given a fixed amount of land, and absent more productive methods, output can be increased only by intensifying land use—increasing the amount of labor per acre—which will necessarily reduce the marginal (and thus average) return to labor.  It is known that land use intensification did occur in traditional agriculture such that the application of labor per acre, and the output per acre, was greatly increased to provide support for a growing population.   At issue is whether innovations and improvements in agricultural technology were sufficient to offset diminishing returns to labor.  If not, then it is very hard to explain why people would have voluntarily changed to a system that required more labor unless forced to make that change because of increased demand, which for most of human history resulted from population growth.
Boserup draws on a large body of literature from anthropology and economics to show how traditional farming evolved in a series of stages from extensive land use to intensive farming, arguing that these stages represent an historical sequence of land use intensification through time.[4]  In the forest fallow system, often called slash & burn or swidden, a forest plot is cleared and cropped for one or two years, after which it is abandoned for 25 years or so until the forest grows back and the method can be repeated.  The bush fallow system is similar, except that the fallow period is shorter—perhaps six to ten years, rather than 25 years.  The shorter fallow period is sufficient only for growth of brush and immature trees.  With the short fallow system, the fallow period is only one or two years, so that when the land is cropped it will be covered with grasses rather than brush.  With annual cropping, the fallow period is a few months between planting seasons.  Under the multi-cropping system, there is no significant fallow period because when one crop is harvested, another is planted; obviously, multi-cropping is normally confined to tropical regions.
Prior to the forest fallow system of agriculture, people pursued an even more extensive land use system—hunting & gathering of wild animals and plants.  [Boserup did not include the hunting & gathering stage in her original model.]   As hunter-gatherers each family required at least a square mile of land to procure subsistence.   Studies of hunter-gatherer peoples show that the time committed to the work of collecting wild plants is quite modest and done almost exclusively by women.  Men spend their time hunting and fishing—activities they enjoy, so they do not consider it to be ‘work’.  
In the wet tropics, the forest fallow system of agriculture can support from five to ten families on land needed by a single hunter-gatherer family.[5]  Clearing the forest will require men to do the work.   Whereas previously men hunt and fish, activities they find pleasurable, men are forced to become involved in ‘work’, that is, activities of procuring subsistence that they do not enjoy.  Fire does most of the ‘work’ and there is no need to remove roots and stumps (which is the more time-consuming work of clearing required when land is used as a permanent field).  The work required to clear two or three acres of tropical forest is estimated to be from 25 to 60 man-days.[6]  In southern Belize Kekchi farmers stated that they would clear their plots in about two weeks, to be completed prior to the short dry period when it would be burned.[7]  In order to get a ‘good burn’ men first cut underbrush and limbs to make a ‘bed’ for the trees that are felled on top.  After a period of drying, the trees are burned, and the ashes provide a thick layer of fertilizer that will insure a good crop.  Seeds are then punched into the ash-covered ground using a digging stick.  There is no need to plow the ground because forest soils are soft and absence of sunlight at the surface mostly precludes the existence of grasses and weeds.  Likewise, there is no need to remove stumps; inasmuch as work is by hand, there is no need to plant in rows.  Thereafter, very little work is required until harvest time.  Often planting and harvesting is done mostly or entirely by women, so that the men can return to their more pleasurable pastimes of hunting and fishing, activities that also provide essential protein to the diet.[8]
The bush fallow farming system is similar to forest fallow except that the shorter fallow period is insufficient to produce mature trees.  The amount of land needed to support five or ten families under forest fallow can support fifteen to twenty families using bush-fallow.  However, brush and immature trees form a thick mass of vegetation that is difficult to penetrate and often more difficult to clear than trees, and the amount of ash fertilizer is greatly reduced, so that it may be necessary to work a somewhat larger amount of land to achieve the same output.   Moreover, because more sun reaches the ground in an immature forest, there will be more grasses and weeds that must be cleared, making it necessary for the farmer to ‘invent’ or ‘discover’ the hoe.  Thus, the family will be forced to invest more labor than was necessary under the forest fallow system to achieve the same level of subsistence. 
Using the short fallow system a square mile of land can support perhaps thirty or forty families.  A fallow period of only one or two years will be insufficient to produce a cover of brush, so that the ground will be covered with weeds and grasses.  Grasses form a thick hard ‘sod’.  Killing grasses (absent herbicides) can be achieved only by ‘breaking the sod’—turning it over so the grass faces down and the roots are exposed.  Thus, the farmer finds it necessary to ‘invent’ the plow, and will be in great need of a domestic draft animal to pull that plow (although as the Inca proved, it is possible to plow without a draft animal[9]).  The short fallow period is insufficient to replenish soil nutrients, so that fertilizer must be added to the soil, which usually means waste from domestic animals and humans must be carried to the field.   Preventing weeds from taking over the field will require the farmer to spend many days during the growing season cultivating his plants with a hoe or plow. 
If the land is used every year, the one-square mile of land can support sixty or eighty families.  And, if the land is used to produce two or three crops every year more than one hundred families might procure subsistence from that land.  However, the more intensively the land is used the greater the need for fertilizer and the greater the need to carefully cultivate the land to prevent weeds.  Under multi-cropping, the one hundred families will find it necessary to have man, wife, and children in the field five or six days each week all year long carefully tending the plants to keep them from being crowded out by weeds.  The high-density, low per capita production, equilibrium in which people work their land as intensively as possible and are barely able to survive and reproduce (epitomized by the paddy-rice culture of past south and east Asia) is sometimes referred to as a ‘Malthusian Trap’.   In fact, this ‘Malthusian Trap’ is perfectly consistent with the Boserup Theory; indeed, it would be an inevitable consequence of population growth unless people adopted technology that greatly increased productivity per person.  That innovation must go well beyond ‘inventing’ a hoe and plow.  Boserup emphasizes distinction between the ‘kind’ of tool, which follows from the level of intensity of land use, and the ‘make’ of tool, which determines how efficient that tool can be.[10]  A cultivator pulled behind a team of horses or a tractor, is essentially a hoe (or series of hoes), but it is many times more productive that a stick with a rock blade attached.
Notwithstanding the potential Malthusian Trap, Boserup observes that the land is far more generous to increased human labor than Malthus thought.  Over the shorter term, people are able to increase the food supply, in spite of diminishing returns to labor, by using more labor-intensive methods and working longer hours.[11]  Even without a technological breakthrough people can realize vast increases in production per acre by working longer hours.   Thus, land sufficient to support one family of hunter-gatherers can support five families using forest fallow; fifteen families using bush fallow, thirty families using the short fallow, sixty families using annual cropping, and more than one-hundred families using multi-cropping.  Nevertheless, diminishing returns to labor is a physical law that will necessarily reduce output per unit of labor unless some new method is applied.  It is important to note that Boserup’s more optimistic view of production increase through intensification is not central to her theory.  Rather, it means that people could accommodate population growth for a much longer period of time by working longer hours to compensate for diminishing returns to labor, thus giving people more time to come up with a technological fix.  Likewise, Malthus’ pessimistic appraisal of potential increases in yield without new technology was not central to his theory; it simply meant that he thought people would hit the carrying capacity sooner rather than later.

The Law of Least Effort

Postulate two applies the Law of Least Effort to land use intensification.  Obviously, people will tend to avoid adopting a new farming system that would increase the labor requirement when the output would remain subsistence production as before.  And, the increased work requirement is only one of the many penalties that accompany the more intensive land use system.  As the number of families per square mile of land increases, the ability to harvest protein from wild animals declines to almost insignificance.  The decrease in protein from wild animals is only slightly offset by the increase available from domestic animals.  Although most traditional farmers have domestic animals, those animals must be fed and cared for, and often they are too valuable to be slaughtered for direct consumption.  The bovines, horses, and donkeys have much greater value as draft animals and beasts of burden than as food.  Pigs and chickens provide some meat and eggs, but consumption of these luxuries is mostly reserved for very special occasions.[12]  Archeological evidence indicates that the evolution of agriculture was coincident with a substantial reduction in the stature of humans, which was not regained until meat became plentiful in western societies.  After about 1960 the average height of the Japanese has increased substantially, probably for the same reason.[13] 
As the fallow period is shortened the problem of ‘weeds’ (mostly herbaceous annuals that have evolved to take advantage of human made clearings) becomes more severe.[14]  Under forest fallow the ground is shaded prior to cutting, and surrounded by forest so that there is little seed supply for ‘weeds’.  Under bush fallow sunlight does reach the ground in many places so that many hours of hoeing is required.  With annual cropping, removing weeds (cultivation) is a major component of agricultural labor.  People who practice multi-cropping (by hand and absent herbicides) find it necessary to have most or all of the family in the field all day almost every day to prevent weeds from reducing or eliminating the harvest.
Intensive farming requires people to become sedentary, so their dwellings will be infested with the ‘animal equivalents of weeds’—mice, rats, roaches, lice, etc.—that have evolved to take advantage of permanent human dwellings.[15]  These pests consume or contaminate human food and spread diseases.  In addition, living cheek-by-jowl with domestic animals provides the opportunity for many diseases to jump from animal to human, with devastating results because humans will lack immunity to these animal diseases.  Smallpox, measles, flu, tuberculosis, malaria, plague, cholera, and HIV, are example of diseases that jumped from animals (mostly domestic, with malaria and HIV being exceptions) to humans.
Finally, the more intensively the land is worked the more it is changed from a diverse natural ecosystem to an artificial mono-culture consisting of a small number of species.  The natural ecosystem almost never fails; thus, people dependent upon an environment that is mostly natural rarely consider the possibility of famine (from natural causes; obviously warfare and disease will produce famine among hunter-gatherers).  When a natural ecosystem is replaced by a few domestic cultigens, crop failure every few years is guaranteed.  Hunter-gatherers almost never experience starvation (caused by natural forces), and they enjoy a great variety of foods including animal protein.   In sum, the disadvantages of agriculture, and intensifying agricultural land use, are so staggering, that adopting that lifestyle voluntary is extremely difficult to explain in the absence of new demand produced by population growth.
It is instructive to note that farmers who have practiced intensive systems will often switch back to a less intensive system if they migrate to a region where more land is available.  The case of the Kekchi Maya, who migrated from highland Guatemala, where they practiced intensive farming, to lowland Belize, where they reverted to slash & burn farming, was noted previously.  Following WWII Japanese farmers migrated to Brazil where they also reverted to a form of slash & burn farming for a period.  It is a simple fact (the Law of diminishing returns to labor) that a farmer can produce food for his family with much less effort by using the land more extensively.  Some would argue that soils in tropical forest regions cannot be used intensively because they lack organic matter and nutrients.  However, people who have worked or conducted research with farmers in tropical regions, as I have, know that is not always true.  The Kekchi Maya in southern Belize live in a tropical forest region and many maintain intensive dooryard multi-story gardens[16] near their home for production of vegetables and medicinal herbs, and some maintain those multi-story gardens in a cleared forest plot.[17]  In the ‘cloud forest’ region just east of the Andes in Ecuador, migrants farmers from the highlands mostly clear the forest[18] and get a single crop of corn; thereafter, they plant alien grasses (from Africa) which will mostly prevent reforestation, and make their living by raising beef cattle.  However, some farmers found rich black soils that would produce a crop of corn year after year.  I was not able to observe any physical difference or cause for those startlingly different soil conditions.  However, recently it has been proposed that in many areas soil conditions, and indeed the forest itself, are anthropogenic—created by farmers who inhabited the Amazon region in very high densities prior to the Spanish invasion.[19]Show more
Why would anyone have ever defined the process of intensifying agricultural land use as a desirable transformation made possible by human ingenuity that allowed people to move up the ladder of civilization?  There are a couple of answers to this question—although they are not very satisfying.  First, intensive farming produces a vast increase in production.  From the Malthusian perspective, that would allow the human population to increase for a period of time.  If human population growth is a desirable goal, then land use intensification can be considered a good or positive turn of events.  However, since that process resulted in lower, rather than higher, production per hour of labor, it is difficult to see how it would constitute an improvement in the human condition.  Second, it is often assumed that the inventions of the hoe and plow, along with domestication of animals, were technological advancements that offset the law of diminishing returns to labor.  That is, annual cropping with a plow and draft animal might result in an increase in output per hour of labor, which would provide the incentive for farmers to make that change.  However, this position is very dubious.  Domestic animals must be fed, and human and animal fertilizer must be carried to the field.  Boserup noted that “the area an animal can till with a primitive plough is much smaller than the area needed to feed it on natural grazing (and therefore) a considerable part of the land must be left as permanent grazing; or the cultivation period must be considerably shorter than the fallow period during which the fields are left to grow wild grasses for the animals to feed on; or, finally, a part of the harvest from the cultivated fields must be given to the animals.” [20]  
Higher output per farm in colonial America (in comparison to Europe) is sometimes cited as an example increased returns per hour of labor.  However, higher productivity was mostly associated with the much lower population densities and thus much larger amounts of land worked by each farmer (resulting from the demise of the native population).  Clearly the introduction of petroleum powered machines did produce vast increases in productivity; however, substituting machines for hand labor would not have been possible absent the vast increase in farm size made possible by migration of the majority of the population to urban areas.  In short, we cannot find evidence to support the view that land use intensification was accompanied by sufficient technological change to offset diminishing returns to labor. 
There are people practicing hunting & gathering and slash & burn farming today who are clearly aware of opportunities to switch to a more intensive land use system.[21]  The Kekchi maintain intensive door-yard gardens, as noted previously.  Among hunter-gatherers in Amazonia, women maintain gardens of cassava and bananas.  Obviously, they know about annual and multi-cropping (personal observation). 
Why would a slash & burn farmer walk two or three miles to a forest plot, and go to the trouble of clearing a new one each year, if they could accomplish the same thing more easily by using a small plot near their dwelling more intensively?  If intensification did not result in diminishing returns to labor, the failure of slash & burn farmers to change to annual cropping could only be explained by arguing that such farmers have land unsuitable for annual cropping (the poor tropical soils thesis) or that they don’t understand how to crop their land more intensively (notwithstanding the fact that many do practice intensive gardening on small plots near their dwellings).  One or both of those arguments is used in most geography, history, and anthropology books to explain why slash & burn farmers cling to their ‘inefficient’ farming system.   Over the past 40 years I have interviewed many slash & burn farmers in various countries of Latin America and can attest to the fact that they will tell anyone willing to listen that they are aware of alternative intensive farming systems but do not wish to make that change because it would require much more work, and there is no market for additional production or no way to get that production to market.  Today that switch can be made with modern tools, hybrid seeds, and access to a market that would produce income to buy a wide array of consumer goods.  Some farmers, especially younger persons, do make that switch today primarily in order to consume manufactured goods, while others continue to resist land use intensification.

Population Growth as a Cause for Innovation

Postulate three states that factors associated with land use intensification and population growth will tend to stimulate innovation.  Boserup argues that higher population densities give rise to conditions that tend to promote technological changes that can potentially (but not necessarily) offset diminishing returns to labor and thus increase wages.  Malthusians often assume that technological change will be more likely when people have lots of leisure time to ‘think things up’ rather than when they are too busy trying to produce enough food to survive.  In sharp contrast, Boserup says higher population density increases the demand for new methods, and provides the motive and opportunity for more division of labor and specialization.  Only under conditions of high population density do systems of writing and formal education evolve, and high densities act to increase the exchange of information.  Hard work gives rise to a ‘work ethic’ and a more energetic population.  According to Boserup the need for higher productivity, resulting from population growth, can be a stimulant to technological change, and having to work hard provides the motivation for change.[22]  Change tends to be self-stimulating and self-reinforcing; the more people adopt new methods the easier it is for them to change again, or think of even better ways of doing things.  In her original work, Boserup focuses mostly on increased demand resulting from population growth.  By extension, the shift of population to the non-farm sector will also increase demand for farm products (assuming the non-farm population produces something of value to farmers) and the much higher densities in the urban sector will also stimulate rapid innovation. 
People who practice intensive agriculture have another advantage over hunter-gatherers and slash-and-burn farmers who live at low density—they excrete germs that are deadly for people who have not lived at high densities or with domestic animals and who therefore lack immunity to those diseases.  Obviously, the farmer’s advantage is a double-edged sword; there is much death associated with acquiring immunity, and immunity is usually not absolute.  However, when farmers clash with hunter-gatherers, the farmers win.  We tend to attribute this to the larger number of farmers and to their superior technology.  However, disease is the most effective weapon farmers have at their disposal, and biological warfare does not require any overt action or even awareness on the part of the farmer.  Obviously, the Spanish conquests of the Aztec and Inca Empires is explained by diseases to which Native Americans had no immunity.  The few hundred Spaniards could not have conquered the Aztec Empire even with modern fire arms and aircraft.  Diseases raced ahead of the Spaniards destroying the vast majority of the population and instilling great fear in the native population.  The germ theory of disease was not known to the Spaniards or natives, so both almost certainly believed that God was on the side of the Spaniards.[23]

Logical Deduction

The logical deduction of the Boserup Theory is that population growth has been the cause, rather than the result, of increased production.  If it is true that people would avoid land use intensification because of diminishing returns to labor, why did that transformation ever come about?  The answer, according to the Boserup Theory, is people will intensify their production systems if, and only if, population growth increases the demand for production on a per acre basis.   That is, only population growth could produce the new demand that would require people to accept the longer work-week and deterioration in life style that accompanies land use intensification.
Thus, population growth is the independent variable and productivity is the dependent variable.  If we were to plot the relationship between population growth and food production on a graph according to the Boserup Theory we would put population growth on the x-axis and food production per acre on the Y-axis to indicate that changes in population growth cause changes in food production.  Furthermore, because higher densities tend to stimulate technological change, population growth becomes the cause for the innovations that could offset diminishing returns to labor and begin to reduce the workweek for the farmer, and other members of society.  It is important to understand that this is the exact opposite of the relationship hypothesized by Malthus.


Both the Malthus and Boserup Theories are grounded on the Law of Diminishing Returns to Labor.  For the Malthusian Theory, that law will restrain population growth and tend to reduce the human population to poverty unless they use preventive checks to stop growth.  For Boserup, the law of diminishing returns explains why people would not have intensified their land use systems in the absence of demand resulting from population growth.
Malthus thought the UK was approaching the limits of production in 1800 when the population was eight million.  The population is now 58 million, and production per person is many times greater than it was in 1800; indeed, expenditures on basic food, Malthus’ primary concern, accounts for only a few percent of total consumption in the UK today.  We have seen corn production per acre increase over the past century from thirty bushels per acre to sixty, and from sixty to one hundred and twenty.  Can it be doubled again to two hundred and forty bushels per acre?  That has already been done on experimental plots; but, could we achieve that as an average yield across large areas of the earth?  The answer seems to be, not unless that becomes necessary because of increased demand.
It seems likely that throughout history most people have told themselves that, whereas past innovations that increased production were rather obvious in hindsight, future innovations that could again increase production, are not apparent, and therefore probably impossible to achieve.   Fortunately however, there were a few individuals that have been innovators and have caused production to increase throughout history, and their numbers seem to be growing.

[1]  Ester Boserup, Conditions of Agricultural Growth: The Economics of Agrarian Change under Population Pressure, Chicago: Aldine Publishing Company, 1965.  Page numbers in the following footnotes refer to Boserup’s book as cited above.
[2] The Law of Least Effort affirms the obvious fact that, absent some desirable return, humans prefer to do less work rather than more work.
[3] Brian Spooner (Editor), Population growth: Anthropological Implications, MIT Press [1972].  See Spooner’s introduction.
[4] Boserup, op cit. pp. 15-35.
[5] A 25 year fallow period is required for regrowth of the forest in most wet tropical regions; thus a maximum of about 25 acres of land per square mile can be cultivated each year.  If each family needs to work from 2 to 4 acres each year to obtain subsistence (personal observation among Kekchi Maya in southern Belize), a square mile could theoretically support from 6 to 12 families; given that some of the land will not be suitable for agriculture the actual number of families per square mile will be somewhat lower.   
[6] Boserup, op cit., p. 30.  Although the work is done by hand, It is not clear what kind or make of tool is used in these estimates. 
[7] I did not observe forest clearing, and understood that men cooperated in that work; thus the number of man-days may have greatly exceeded a one-man two-week requirement.  I visited the Kekchi Maya community annually from 1973 to 1990 but most unfortunately did not ask all the questions or keep good notes (it didn’t seem so important at the time).  I never observed an axe (but can’t affirm that they did not have them); all men carried machetes with a file in their back pocket, used periodically to keep the machete razor sharp; indeed, they would wear out several files before one machete was reduced to a ‘kitchen knife’.  During the 1970’s almost all families practiced forest fallow agriculture, although many maintained intensive gardens near their homes and some practiced annual cropping along that portion of the local stream that flooded annually leaving a rich layer of silt, which they called their ‘mata hambre’ (kill hunger) crop, as a reserve in case the forest crop failed.  It must be noted that the Kekchi would not be representative of forest fallow farmers.  The Kekchi migrated to southern Belize from highland Guatemala where they practiced a far more intensive farming system; thus ‘work’ had become a normal activity for men and they had a strong work ethic.  Nonetheless, men had considerable leisure time and could spend many days fishing and hunting (or accompanying silly foreigners through the rain forest and responding to questions about the plants and animals, Maya ruins, plant-animal symbiosis, etc.).
[8] Among the Kekchi Maya, men do much or most of the work required for planting and harvesting.  As noted previously, men developed a strong work ethic in highland Guatemala where they used their land far more intensively.
[9] The ‘Inca’ hand-plow system was still in use in some areas in the early 60’s when I served as a Peace Corps Volunteer in a village near Cuzco.  It required three men, using spades with a foot peg allowing the spades to be pushed into the ground with foot pressure.  The three men worked on three sides of a single, approximately one square foot, block of earth, turning it over.  The next block of earth is turned over into the depression made by the first.  I did not attempt to calculate how much land could be tilled by three men per hour, but they did move along faster than I had thought possible.  Of course it is very hard work, and would be done only when a plow pulled by a domestic animal is not available.  In most cases native farmers used oxen to pull a wooden plow tipped with a steel blade from a truck spring.  Prior to the Spanish conquest, the Inca had no oxen, horses, or donkeys, so that the hand plow system would have been required in all cases.
[10] Boserup op cit., p. 26-27.
[11] Boserup op cit., p. 14.
[12] Personal observation in various regions of traditional agriculture in Latin America.
[13] While teaching in Japan in the mid-1980’s I was struck by the fact that college students were usually taller than most adults, and high school students were generally taller than college students.  Note that college students were growing up just as incomes (and therefore consumption of meat) were increasing in Japan, and high school students had generally consumed almost as many hamburgers as American kids.  I must emphasize ‘almost as many’.  I recall a single student in my classes (in Chubu University in Kasugai) who was overweight (obese), and that individual being a constant question among other faculty members.  Why would a college student be overweight?  I have not been back to Chubu University, but I would be willing to bet that an overweight student is not so rare today.
[14] Notice that humans are the only animal that clears relatively large areas of forest or grassland, and thus provided the primary opportunity for ‘weeds’ to evolve and take advantage of those clearings.
[15] Humans are not the only animals that make dwellings, and thus not the only animals that are plagued by ‘the animal equivalent of weeds’.  However, humans have far more elaborate and more permanent dwellings so they provided a far better environmental niche for that sort of symbiotic evolution.
[16] In tropical regions natural vegetation occurs in multi-stories; that is, the forest is often layered in three distinct stories with immature trees and vines (lianas) capturing any sunlight that reaches the surface.  Farmers wisely copy that arrangement with tree crops, bush crops, and ground crops arranged to prevent sunlight from reaching the surface (which would encourage undesirable herbaceous foliage to grow).   
[17] Personal observation.
[18] In the cloud forest region it is not possible to burn the felled trees because there is essentially no dry period.  Farmers simply cut the trees and allow them to decay (a process that takes only about two years in that hot-wet region).  They can, nonetheless, plant their corn around the felled trees and achieve a good harvest that first year (personal observation).
[19] William Denaven, “The Pristine Myth: The Landscape of the Americas in 1492," Annals of the Association of American Geographers, 1992.
[20] Boserup, op cit., p. 35.
[21] Many Kekchi Maya farmers who practiced forest fallow maintained a hunting stand at the edge of their plot which was used to hunt (using a shot gun) wild animals (mostly a forest species of peccary, and tepezcuintle, a rodent the size of a large house cat) that come by at night to eat the crops.  I was not able to determine whether the primary purpose was to protect the crop or to procure the meat.  Some of the farmers plant a special crop around their plot meant to attract those animals; again, I do not know if the reason was primarily to discourage the animals from eating the main crop or to attract them.  When they killed an adult animal that had young, the young were gathered when possible and brought back to the village to be raised for food.  These animals are obviously semi-domestic; the peccary were simply confined with (European) pigs that seemed to get along fine with them.  Young tepezcuintle were penned separately and fed, but would sometimes reproduce in captivity.  Whatever the primary reason, these animals provided a very tasty meal. 
[22] There are several places in Latin America where a national border separates a population that has lived at high density for hundreds of years from a population that lives at low density.  The El Salvador/Honduras border is one case and one can see the same contrast when traveling from highland Guatemala to lowland Belize.  The differences between the high density and low density cultures are absolutely startling.  Hondurans and Belizeans are laid back and mostly relaxed (I don’t use the term ‘lazy’ because their attitude is perfectly understandable).  Salvadorians and highland Guatemalans are in a hurry; one of them can usually do the work of a half-dozen people from a low density country.  And they should be in a hurry; they may not have food to eat tonight!
[23] Native Americans mostly adopted the religion of their conquerors; that has certainly not been the norm throughout human history.  However, one can imagine the impact of having a small number of strange humans invade holding up a Cross and witness the majority of your population die within a short period.    

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