The Mathematics of Migration: Predicting Flows and Forecasting the Future

Benjamin Fields

Content Objectives

Essential Questions to Answer

There are four major families of questions to answer that will help us understand the puzzle of migration flows in the contemporary world. The first most important is through the following question: how do societies develop in terms of population, and what are the most important factors in this development? This is an important question. As humans have developed throughout history, different factors have changed to affect our population. Health, education, and nutrition are the most salient examples. Depending on views of science, humans started out as hunter-gatherers a few hundred thousand to two million years ago. We had certain life expectancies and trends in birth and death rates. Many assume that living “natural” is perfectly healthy, yet when we did, we did not live long, and the quality of life was poor. However, we had transitions in our ability to address death (health interventions and grow food to nourish ourselves in abundance) and make things easier for ourselves to do this, to survive and thrive (technology). Students need to understand this to understand why we have technology, agriculture, and other important modern industries, and how to improve and iterate on them in the future, and also what these mean for the human race and themselves and community. Economy is very important to this, as it dictates many things such as access to healthcare, education, investment in education and healthcare, investment in food systems, and many more concepts.

The second important question is what are the current trends in population change in the United States? What factors are influencing this? We all live in the United States and our quality of life depends on what is going on in the country. Trends in population change have a huge effect for many reasons. For example, on resource allocation from public entities (think building infrastructure, having hospitals and schools), how much we are taxed (which affects our financial state and subsequent wellbeing), and more. The natural increase in population changes for many reasons which will be discovered in the previous question, but it is heavily related to women’s rights and gender inclusivity. Furthermore, there is migration, which is affected by a myriad of factors. Economy is one of them, and you can see this for example most prominently in the rural to urban transition due to more opportunities financially.

The third question is what future changes can we expect to see in the population when we think about the United States? This requires understanding the factors that affect population change including both natural increase and migration, and then being able to synthesize information about current events, trends, technology, and other schema to forecast what population change should look like under certain parameters and scenarios.

The final question is how can we be better equipped to benefit from this ourselves? This is ultimately the most important piece. How can students think of future trends and operationalize them into something useful for themselves? How should they pick a career in the future? How should they decide where to live? How should they decide where to invest?

Unit Content

Due to the nature of the complexity of understanding future demographic changes and what we should do in their wake, we need to have a significant amount of background experience. As the instructor, I have a diverse array of knowledge that is beneficial towards tackling this problem. The primary apparatus we are going to use to explore the questions listed above is the Demographic Transition Model listed in Figure 1.

Figure 1: The Demographic Transition Model from Our World in Data[DT1] , heavily detailed with information explaining the trends and includes population pyramids.[i]

This model gives us a way to understand how different societies develop over time with marked changes in the two factors of natural increase in population, that is based on the differences between the birth rate and death rate of the indigenous population absent of migration. On top of this model, we can map other models that affect the birth and death rates. For example, the Epidemiological Model[ii] [DT2] helps us understand why in the beginning stages the death rate drops before the birth rate. It gives us an understanding of the different causes of death over time. We start off by dying from communicable diseases that transition to harder-to-solve non-communicable and degenerative diseases like heart disease and cancer. The Nutritional Model[iii] [DT3] builds upon that, giving us a framework of how humans fed themselves over time. We started off as hunter gatherers, developed agriculture, and intensified it over time until we reached a point, we developed more processed methods for food. Lastly, we have the Technological Model[iv] [DT4] which outlines how different improvements in technology at different stages led to advancements in managing the birth and death rates. This includes health technology, productivity technology, and more. Additionally, you must understand social trends and how they lead to behavioral changes that affect the birth rate. Women’s education and workforce participation rate, products of social movements, help explain the reduction in birth rate[DT5] .[v]

Other important factors are geography, geopolitics, and international relations. Things that affect the population are broad, especially related to migration since the United States currently has over 38.5 million migrants, most of the people are immigrants from past generations[DT6] [vi], and it is a pivotal piece of our history and current events. This includes nuances that involve geographic knowledge around the world and different places. For example, we should be familiar with countries that are North and South of us. We should know that there are groups of people who migrate up through the southern border due to them being connected to Mexico and there being pipelines to facilitate this whereas to the North[DT7] [vii], it would be hard for someone to get into Canada and then come down to the United States. These are not moral or political arguments, but just facts about the situation. The United States needs and wants this migration, but there are complications that prevent this from being shared en masse. This segways us into geopolitics and international relations. There are so many things going on with the U.S., an emerging (theoretical) hegemonic power like China, and the possibility of places like the Gulf States, BRICS, or other coalitions coming to change the world order. Americans are unaware of many things in government[viii], which makes it likely they are unaware of geopolitics, international migration, or threats to the country’s national security. It is important to be aware of these when projecting the future, and as we see throughout the background knowledge needed to run this unit, you need to have as wide a scope of view as possible. There is a journalist by the name of David Epstein who wrote a book called Range: Why Generalists Triumph in a Specialized World[DT8] [ix]. He cites research that shows the more specialized someone is in one narrow field, the more inaccurate their predictions were about the world. This is why I promote being as well rounded by proficient in as many topics ancillary to our learning goal as possible.

Next, we have current events and migration trends. There are many things going on in the world right now. There are wars, new elections, nationalist movements, climate change, and a myriad of other factors that affect quality of life, where people want to be, their opportunities, and more. These are going to be very important in understanding future migration trends and how students can use this knowledge to their advantage. They are also understanding migration trends by themselves. There are certain theories of migration that hold significant weight. There is push and pull theory (the idea that certain factors can attract people to a location or that some factors can repel people from a location)[x], [DT9] Ravenstein’s Model (a model that posits a few factors: that most migrants only go short distances, that migration goes in steps towards the final destination, that long distance migrants prefer big cities, that people from rural areas migrate more than those from urban areas, that females migrate inside of countries more while males migrate internationally, that most migrants are adults and not full families, and that urban areas grow mostly from in-migration and not natural increase)[DT10] [xi], and gravity theory (that people generally tend to migrate to places that are large and/or already have a strong amount of migration[DT11] )[xii]. These will influence how migration unfolds given all the external factors like technology, freedom, financial ease, and more. I also want students to know where migration is taking place. For example, most of the migrants to the United States are not from Mexico, but from other places, and there is a net outflow of Mexicans from the United States instead of inward[DT12] [xiii]. Furthermore, there is a significant amount of migration to globalized business hubs in places like the Gulf, Southeast Asia, cities in Australia, and other places. There are refugee movements due to wars, and other important trends to note that will influence migration. There is even research suggesting that millionaires within the United States do not move for lower taxes[xiv], and that wealthy people are concentrating in certain countries like the United Arab Emirates, the United States, Canada, and Singapore while leaving places like China and authoritarian regimes[DT13] [xv].

Very importantly is understanding the economy, both globally and domestically, and including the new economics hubs and global business environment. The economy is a complicated subject and involves many moving pieces. But for starters, students should understand what gross domestic product (GDP, the total value of all goods and services that is created within an economy in a year), and other related figures are. These helps measure the size and strength of economies. This just be done on an aggregate level as well as a per capita basis. Students should also have an understanding of what imports (goods being brought into the country) and exports (goods being taken out of the country) are, remittances (money sent from individuals in one country to another), taxes, and other economic terms that look at the flow of goods over borders, measure how great economies are doing, show where business is thriving, and even measure flows of money between groups. Remittances to Mexico are going to tell a story since they have a large diaspora here in the United States. There should be an understanding of remittances within the United States but also globally. You should have a basic idea where major industry products like technology, clothing, and even agricultural products are produced and what effect this has on the economy and consumerism. Lastly, there should be an understanding of economic hubs and new business environments. Switzerland used to mean something, Panama means something, tax havens mean something, and the newly emerging Gulf States as well as places like Singapore mean something. These all will affect migration, the future global economy, and have nuances for students.

The future of the population is unsure. While we know the global population is increasing significantly[xvi], we do not have reliable estimates and predictors for the future. We could undergo population growth due to declining populations figuring out how to convince their women to have children, or we could go into an era where developing economies do successfully figure out a way to reduce their birth rates if they find a compelling reason to do so. To finish the unit, students should think about population growth and which careers will be most in-demand and suited for them. For example, there will be growth in the U.S. healthcare sector as migration continues, we will need more housing, and more.

Parts and Subsections

The unit is designed to be two weeks long. There are four parts that will take about 2 days each. Since these are contained within a class on human geography, students will have enough background knowledge to not need extensive clarification and background knowledge, speeding up the lesson. Part 4 is the only place that students may not have formal familiarity, but they do have personal familiarity at least with the cost of living and personal nuances.

Part 1: Understanding the Demographic Transition Model

• Natural Population Change
  • Birth Rate
  • Death Rate
  • Population Change
  • Population Pyramids
• Ancillary Models of the Demographic Transition
  • Health
  • Technology
  • Nutrition
  • Social Movements

Part 2: Migration

• Major Theories
  • Push and Pull
  • Gravity Model
  • Ravenstein’s Model of Migration
• Trends
  • Global Population
  • American Population
  • Global Migration
  • American Emigration
  • American Immigration

Part 3: Global Factors

• Geopolitics
  • International Relations
  • Regime Theory
  • World Systems and World Society
• Current Events
  • Economy
  • Technology
  • Industry
  • Religion
  • War

Part 4: Financial Factors

• Real Estate
• Non-Real Estate Economy
  • Interest Rates
  • Investments
  • Cost of Living
• Personal Nuances
  • Future Occupational Choices
  • Lifestyle Preference

Additional Background Knowledge

Students need skills to be aware enough to make proper decisions for themselves in the future. Most students in our high school context have not left their hometown for a significant amount of time; this has limited their point of view as to what is available in the world and what they can participate in. For example, most students do not have a comprehensive view of the amount of career paths that exist, or what it is like to live in a big city, or what it is like to have a strong network, or what a business is like in a large major metropolitan city. Thus, students need some exposure to this to be open and receptive to the unit. Furthermore, students do not have exposure to the proper information that dictates homeownership. For example, understanding principal and interest, property taxes, HOA fees, homeowners’ insurance, maintenance, security costs, and more.

Teaching Strategies

Flipped Classroom

I plan to use a flipped classroom strategy. This means I will assign videos for students to watch at home before we meet in class. Afterwards, we will start with a brief recap of the readings guided by an open class quiz we all take together and answer the questions out loud. After we finish addressing the questions, we are going to go through guided discussion questions posed to bring out nuances and students’ ideas from the videos. This will give students a chance to be creative and share their ideas with their peers. The final step will be me answering any questions they have after the lesson and correcting any mistakes in their analysis or understanding.

Flexible Seating and Groups

I also plan to use flexible seating and groups. Most particularly, I have a SmartBoard as well as 2 walls worth of whiteboard space. Desks will be positioned so most of our work is done on the whiteboards together in groups, and desks will be grouped in 4s so students can work with each other. This will be all throughout the lesson, and there will be no individual work.

Peer Instruction

One strategy is to allow the students to educate each other. So, when we are answering questions for the flipped classroom beginning part of the lesson, the questions will be categorized by theme and the different tables will have to answer and educate the rest of the classroom on the questions. This will help humanize some of the topics and allow students to hear an explanation that is not from the perspective of a much older teacher.

Games

Having games and gamified activities will be pivotal in keeping students engaged and enjoying the lesson. Due to challenges post-COVID, attention span issues are real. If we can mimic some of the things they watch on social media and take their addictive elements and put it into the curriculum, that will yield some success. Each day, the warm welcome/bell ringer will be a game and the exit ticket will be a game. There was a technology I used in university called i-Clickers that can be used, and their participation in these i-Clicker activities can be logged for a participation grade. Matching games, hangman with relevant terms, and more can all be unique ways to get students engaged and keep them engaged. They will also retain more content.

Data Analysis

Lastly, we are going to do a significant amount of data analysis and look at different relevant data. Here are the parts and subsections again, each with the type of data we are going to use and the possible analysis we would look at:

Part 1: Understanding the Demographic Transition Model

• Natural Population
  • Birth Rate: We are going to look at data[xvii] [DT14] on the number of births per year in the country and in certain states/regions of the country as well as the rest of the world. We typically look at birth rate per 1,000 women; after this, we can look at it over the reproductive years of women to determine the fertility rate, which is the average number of children a woman has over her reproductive lifetime.
  • Death Rate: We will look at life expectancy, which is based on age-adjusted death rates for all ages[DT15] [xviii]. There is nuance to the death rate in related population health terms like health-adjusted life years and disability-adjusted life years. We will measure how many years of health life individuals are living.
  • Population Change: We are going to look at how the population has changed from 1900 until today for the United States and other countries as well as regions and globally. We will also look at projections until 2060[xix].
  • Population Pyramids: We will discuss population pyramids and understand the age structure of countries. For example, the United States has a medium age structure where there are plenty of old people but still a significant amount of young people. Countries in Africa have very young age structures where over ¾ of the population is under 25 years old, and Western Europe has a slightly older age structure, especially Italy and Spain, which is why they were hit so harshly during COVID.
• Ancillary Models
  • Health: We are going to look at the major causes of death (without a focus on nutrition) over time, understand basic epidemiological principles like case fatality rates, incidence, and prevalence, and more.
  • Technology: There are important rules like Moore’s Law and others that give insight into how fast technology changes, and we can chart this out into the future to understand how things are going to change.
  • Nutrition: Understanding the major causes of death over time related to nutrition and how our current food systems work in this. For example, looking at the amount of time it takes to produce food (it used to take months to grow chickens, now it can be done in weeks).
  • Social Movements: Important dates, times, and predictors of social movements including those surrounding women and gender and other human rights issues.

Part 2: Migration

• Migration
  • We are going to look at the migration data[xx] [DT16] and how things are changing over time. This includes migration across the United States border specifically, rates of international students studying here in America and other countries, and most importantly, on data from the United Nations Development Program (UNDP)[xxi]. [DT17] The UNDP releases data on international migration stock in terms of origin and destination, having percentages of the population at mid-year and the absolute numbers at mid-year, also broken down by gender.

Part 3: Global Factors

• Geopolitics
  • International Relations: We are going to construct a rudimentary timeline of major events globally in terms of international relations and make sense of them.
• Current Events
  • Economy: We are also going to compare the size and scope of economies, imports and exports of different countries, and other factors. This includes looking at GDP, GNP, and other terms both holistically and on a per capita basis.
  • Religion: How changes in religious structure and different birth rates in different countries affect this, and how it affects migration and structure. We are going to look at countries with the fastest growing rates and their religion and look at the demographics most likely to migrate.
  • War: We are going to look at the number of wars, the amount of money spent, and other numeric factors to understand this and how it affects migration. For example, there are numerous refugees from the Ukraine war. There are also political and economic ramifications in the United States that may affect migration or have a confounding fact on migration through another factor.

Part 4: Financial Factors

• Real Estate: Average real estate prices over time, average mortgage interest rates over time, and have a broader idea of how much real estate costs across the world in different regions and select countries. Also delineate this based on rural vs suburban vs urban properties.
• Non-Real Estate Economy
  • Interest Rates: How interest rates work and how it affects the economy, migration, cost of living, and more.
  • Investments: How investments work including ones like social security (which is technically not an investment but one that is forced from the government) and how this impacts quality of life based on where you live. This also includes understanding a little bit of tax.
  • Cost of Living: There are websites that create neat reports that compare the cost of living, relate it to income, and break it down by product all the way down to different commodities and consumer goods.
• Personal Nuances
  • Future Occupational Choices: We are going to look at projections for the largest growing careers and industries and where students can take advantage based on what we are learning in the unit.
  • Lifestyle Preference: We are going to do a deep dive into income and cost of living. Most students are not familiar with how much money adults make, what social class income cut offs are, or even about deductions and other relevant financial information that is pivotal in defining lifestyle and preference.

Classroom Activities

When referencing the flipped classroom strategy, the students will watch all the introductory videos for each concept beforehand with the videos being very brief (2 minutes). This will allow us to use peer instruction at the beginning of class. A bank of questions will be developed that students will answer in groups by going up to the board and writing under the section’s summaries of each concept. This will identify gaps in the knowledge and allow students to inform their peers and brainstorm their thinking and understanding of the different topics.

Afterwards, we will engage in moderated discussion and debates about connections across the concepts, current events, and what we can expect in the future given the previous trends. This will allow students to explore connections between the past which are important and what we want them to understand, but to also relate them to today and tomorrow since these are usually the types of things students want to know and the frames of reference that are excluded most in educational settings.

We will also have games specifically tailored towards the research . For example, we will match population pyramids with different countries at different stages of development, have worksheets where students fill in blank maps with different colors based on what they suspect the population density to be, and unique internal classroom environments where we synthesize parts of the classroom with different attributes and see how students migrate between them physically.

Resources

Here we will look at a few of the images and schematics that students will be given in class to make sense of the unit. This includes YouTube videos, texts, and most importantly, images.

Demographic Transition Model Video: This will be an introductory video that introduces students to the Demographic Transition Model and how population changes over time, at least in the context of natural increase, due to birth rate and death rate. We will discuss in-class the ancillary models and model them on the white board with markers.

World Systems Theory Video: World-Systems Theory is a broad, multidisciplinary framework for understanding social dynamics and global history. It posits a world-system instead of individual counties being the central unit of analysis. It states that the world is an interconnected system, where economic, political, and cultural forces transcend borders, influencing societies on a global scale. The main theorist behind it is Immanuel Wallerstein who explains in detail how core nations dominate and exploit peripheral ones within this global structure. The World-System Theory focuses on the role of capitalism, disparate power relations, and long-term historical trends in shaping the world. It offers valuable insights into the complexities of globalization and social change.

San Diegans Living in Mexico to Save Money Video: Here is a video that shows how many people who are from San Diego move across the border to Tijuana to live while they work in San Diego. This will yield an interesting conversation around quality of life, cost of living, economy, migration, and more. It will also engage the students as it is applied, related to the current day, and portrays a somewhat international lifestyle that they would find attractive.

Visual of the Demographic Transition Model: Students will also receive a visual of the Demographic Transition Model created by Our World in Data (see figure 1). This one is particularly useful because it includes the birth rate, death rate, population, and also numerical explanations about population change and even population pyramids!

Visual of the Change in the Mexican-Born Population in the United States: Lastly, students will learn about the trend of the Mexican-born population in the United States. While the media and other popular conception’s view Mexicans flowing across the border, that is not true. There is a net outflow of Mexicans leaving the United States!

Figure 2: A Graph Estimated the Amount of Foreign-Born Mexicans in the United States

Appendix

This unit matches very well on a multitude of standards for World Geography in terms of the State of Oklahoma or AP Human Geography at the advanced level. Focusing on the standards for Oklahoma, it addresses the following:

WG.1 The student will use maps and other geographic representations, tools and technologies to acquire, research, process, and solve problems from a spatial perspective.

• We are going to use geography representations of the globe to show how weather and climate can drive migration. Previously, people did not know about tropical places and did not have a means to visit them. Now, it is easy for people to do this, not just on a temporary basis, but permanent due to things like remote working and freelancing. We are going to explore the globe, the positionality of the Equator, and the places that people migrate to most.

WG.2 The student analyzes how the human population is organized geographically in order to understand the cultural, political, and economic systems of the world.

• We are going to use theories in international relations like world systems and world society to understand how different hubs of humanity (for example the West vs China vs Arab) organize themselves culturally, politically, and economically, and how this affect migration of the human population, especially in relation to the United States. 

WG.3 The student will analyze the components and regional variations of cultural patterns and processes.

• Right now, global culture is complicated. There are international relations core theories that describe a global culture being created through the Western world’s domination over the previous few centuries, but we will also discuss the emergence of different cultures around the world rising to prominence and how they affect migration. For example, Mexico is becoming a powerful economy, and their citizens have a significant presence in the United States. This affects the country’s culture as well as propensities for certain demographic groups to migrate.

WG.4 The student will explain the political organization of space.

• We will discuss how political boundaries affect migration across borders locally and internationally.

WG.6 The student will analyze the impact of industrialization on economic development.

• We will discuss the impact of countries around the world developing and how that affects migration flows.

WG.7 The student will evaluate specific textual and visual evidence to analyze cities and urban land use.

• We will look at maps that describe the flow of people moving inside and outside of cities both internal to countries and internationally.

Annotated Bibliography

Beier, M. E., & Ackerman, P. L. (2001). Current-events knowledge in adults: An investigation of age, intelligence, and nonability determinants. Psychology and Aging, 16(4), 615–628. https://doi.org/10.1037/0882-7974.16.4.615

• This will show the importance of staying current with what is going on in the world and how it can affect the student and their development and the future of migration.

Exploring migration causes: Why people migrate | News | European Parliament. (2020, January 7). https://www.europarl.europa.eu/news/en/headlines/world/20200624STO81906/exploring-migration-causes-why-people-migrate

• Covers the major rea

Frost, M. (2022). Thomas Malthus, Essay on The Principle of Population. In Crossref. Routledge. https://doi.org/10.4324/9780429355653-83

• Many students in our particular context (low-income) have a scarcity mindset which affects their view of migration and travel. This will hopefully open up their eyes to potential in different pockets in the world, and give insight as to why people see some places as land of opportunity and places to migrate, enhancing their understanding.

Gonzalez-Barrera, A. (n.d.). Before COVID-19, more Mexicans came to the U.S. than left for Mexico for the first time in years. Pew Research Center. Retrieved January 21, 2024, from https://www.pewresearch.org/short-reads/2021/07/09/before-covid-19-more-mexicans-came-to-the-u-s-than-left-for-mexico-for-the-first-time-in-years/

• This will give students insight that migration is not black and white and true to narratives! There are nuances, and actually more Mexicans leave the U.S. than enter. This will lead to fruitful conversation since it may relate to a significant amount of student experiences and does not necessarily have negative connotations.

James Holzknecht (Director). (n.d.). Maslow’s Hierarchy of Needs. Retrieved January 21, 2024, from https://video.alexanderstreet.com/watch/maslow-s-hierarchy-of-needs

• It may be difficult to explain to students how important the economy, quality of life, and many things are needed in order for a person to be content. Students can work in high school and earn money, but do not need to purchase necessities at the bottom of the pyramid. However, after they become independent and live on their own, their spending shifts towards lower rungs of the pyramid. This is an attempt to get them to understand that, which will also help them understand why people migrate if  they can’t reach certain points.

       Lay, J. C. (2006). Learning About Politics in Low-Income Communities: Poverty and Political Knowledge. American Politics Research, 34(3), 319–340.

Massey, D. S., Arango, J., Hugo, G., Kouaouci, A., Pellegrino, A., & Taylor, J. E. (1993). Theories of International Migration: A Review and Appraisal. Population and Development Review, 19(3), 431–466. https://doi.org/10.2307/2938462

• This paper is useful because it summarizes the different theories of migration and tries to make sense of them. It is a great optional reading for students who want to take a more academic approach and possibly do an extracurricular research project on the topic.

Natarajan, A., Moslimani, M., & Lopez, M. H. (n.d.). Key facts about recent trends in global migration. Pew Research Center. Retrieved January 21, 2024, from https://www.pewresearch.org/short-reads/2022/12/16/key-facts-about-recent-trends-in-global-migration/

• This citation is useful because it gives an outside perspective on the data we are going to review from the UNDP. We are going to look at the data specifically outside of context and analyze it, but the Pew Research platform will connect it to current events, international relations, and summarize the geography as well. It also includes more nuance like remittances, gender, and refugee status.

Nations, U. (n.d.). Human Development Index. In Human Development Reports. United Nations. Retrieved January 21, 2024, from https://hdr.undp.org/data-center/human-development-index

• This citation is relevant because it helps us understand the different levels of development between countries. This will be a major factor in understanding migration and why someone would want to move in or out of a country, or from one country to another.

Roser, M. (2023). Demographic transition: Why is rapid population growth a temporary phenomenon? Our World in Data. https://ourworldindata.org/demographic-transition

• This citation is relevant because it is the main theory we are going to be working from: the Demographic Transition Model. It is a very detailed one as well as it includes the visual graph with the states, birth and death rates, but also describes them and includes useful elements like population pyramids.

United Nations Population Division. (n.d.). International Migrant Stock | Population Division. Retrieved January 20, 2024, from https://www.un.org/development/desa/pd/content/international-migrant-stock

• This citation is relevant because it gives direct data about migration to and from the United States and even gives regions and specific countries. This will provide real time data over time as it is updated every five years starting in 1990 up to 2020. We are going to use data analysis, or at least review data analysis, that shows these trends and try to explain them in light of everything else in the unit.

Written Permissions

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Notes

1. [i] Max Roser, Demographic transition: Why is rapid population growth a temporary phenomenon? https://ourworldindata.org/demographic-transition
2. [ii] Mackenbach JP. The epidemiologic transition theory. J Epidemiol Community Health. 1994 Aug;48(4):329-31. doi: 10.1136/jech.48.4.329-a. PMID: 7964327; PMCID: PMC1059977.
3. [iii] Adam Drewnowski, Barry M. Popkin, The Nutrition Transition: New Trends in the Global Diet, Nutrition Reviews, Volume 55, Issue 2, February 1997, Pages 31–43, https://doi.org/10.1111/j.1753-4887.1997.tb01593.x
4. [iv] Michael Grimm. (2003) Family and economic growth: A review. Mathematical Population Studies 10:3, pages 145-173.
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