The Demographic Transition: Growth or Decline?

A shift in the history of human populations

The demographic transition refers to the gradual shift of a population from a traditional demographic regime—characterized by high birth and death rates—to a modern regime, where both rates are low. This phenomenon is closely linked to industrialization, and to sanitary, medical, and technological progress. It generally breaks down into four phases:

• Phase 1: high birth and death rates; low growth; typical of pre-industrial societies.
• Phase 2: mortality drops rapidly due to medical progress, but birth rates remain high → rapid growth.
• Phase 3: birth rates begin to decline, notably through access to education and contraception.
• Phase 4: low birth and death rates → stabilization or decline of the population.

A universal but non-homogeneous phenomenon

All human societies seem to go through this transition sooner or later, but the timing varies greatly from continent to continent. Western Europe began this transformation as early as the 18th century, while some sub-Saharan African countries are still in phase 2 or 3 today. This time lag generates significant geopolitical dynamics: migratory pressure, unbalanced urban growth, and resource tensions.

An irreversible transition and its consequences

The demographic transition acts as an irreversible phase change in a complex system: once the mechanisms are triggered (control of mortality, control of births, changes in social behavior), a return to the previous regime becomes highly improbable, like a thermodynamic state change. This structural shift profoundly transforms the balance between births and deaths, thus modifying the pace of evolution of human societies.

One of the major consequences is the aging of the population. In physics, we would say that the "population" system increases its temporal entropy: the proportion of older states grows with time. In a high birth rate society, the age pyramid is wide at the base. But as soon as fertility falls, this base narrows, and the proportion of older individuals grows mechanically. In France, for example, the proportion of over-65s has risen from 10% in 1970 to over 21% in 2025. This shift directly affects the dependency ratio: the number of inactive people per active person.

Another effect: the decline in the fertility rate. This parameter is a macroscopic marker of the reproductive dynamics of a living system. The generational renewal threshold - often set at \( 2.1 \) children per woman - is no longer reached in most developed countries. This phenomenon results from the generalization of access to education, particularly for women, the spread of contraceptive methods, the delay in the age at first childbirth, and contemporary economic trade-offs (cost of children, job instability, urbanization).

This demographic slowdown is also reflected in a decline in the natural growth rate. On a global scale, the annual demographic growth rate has fallen from over \( 2\% \) in the 1960s to around \( 0.8\% \) in 2025, according to United Nations projections. Some regions are already experiencing natural demographic decline, particularly Eastern Europe and Japan, a phenomenon still offset elsewhere by the growth of young populations in sub-Saharan Africa.

The implications of this dynamic are systemic:

• Economic: imbalance in pay-as-you-go pension systems, decline in the working-age population, slowdown in domestic demand.
• Health: increase in age-related chronic diseases, increased pressure on health infrastructures.
• Territorial: reverse rural exodus, with city centers deserted by young people and reconfigured around the needs of the elderly.
• Technological: increasing use of automation and robotization to offset the labor shortage.

This process is also accompanied by an urban planning transition. Megacities are absorbing the majority of the residual demographic growth, leading to local overcrowding, increased energy flows, and water stress. This puts a strain on infrastructure systems (housing, transport, sanitation), causing runaway effects analogous to the positive feedback observed in nonlinear systems physics.

Finally, the demographic transition marks a profound cultural change. The child is no longer an economic necessity or a social obligation, but a personal, considered choice, often deferred. This shift in the relationship to human reproduction is unique in the biological and sociological scale of the species Homo sapiens. By modifying the fundamental parameters of reproduction and survival, the demographic transition redefines the life cycle, social organization, and economic structures.

Towards a fifth phase?

After the classic four-phase demographic transition, some demographers and social science researchers are now talking about the emergence of a fifth phase characterized by sustained demographic decline in several advanced societies. This phase is distinguished not only by very low fertility, but also by a new socio-economic and biological dynamic, associated with several interdependent factors.

• Deep cultural changes: reproduction is becoming an increasingly deferred or limited personal choice, influenced by post-materialist values, the empowerment of women, and the spread of urban lifestyles.
• Economic constraints: the direct and indirect cost of education, health, housing, and job insecurity reduce the desire or ability to have several children.
• Biological and social effects: the increase in the average age at motherhood leads to a decrease in biological fertility, combined with an increase in life expectancy.

The systemic consequences are significant and can be compared to a dissipative system out of equilibrium that tends towards a new stationary state, characterized by a stable or declining population:

• Imbalances in social systems: pension funding, health coverage, increased dependence of older generations.
• Reconfiguration of economic dynamics: scarcity of the active workforce, need for technological innovation (robotization, AI).
• Geopolitical impacts: increased migration to compensate for the demographic deficit in some countries, social and identity tensions.

This phenomenon also raises fundamental questions about the sustainability of the global demographic growth model, the ability of societies to adapt to declining populations, and the role of public policy in stimulating or regulating demographic dynamics. Thus, the fifth phase could represent a new stage in human socio-biological evolution, marked by a complex interaction between biological, environmental, economic, and cultural factors.

Terminal shock between climate change and demographic decline

The interaction between global climate change and the demographic decline underway in several regions of the world can generate unexpected systemic risks, otherwise known as a terminal shock affecting global socio-economic and ecological stability. These risks result from complex nonlinear feedback, characterized by runaway phenomena coupled with structural limits in human adaptive capacities.

From a physical point of view, this shock can be conceptualized as the convergence of two dynamic processes with different but strongly interdependent time scales:

• Climate change: rapid evolution of global environmental variables, such as average temperature, atmospheric CO₂ concentration, or sea level.
• Demographic decline: gradual but inexorable decline in population in certain regions, reducing socio-economic resilience and adaptive capacity.

Unexpected risks

Among the specific risks identified, beyond the well-documented threats, are phenomena not anticipated by human intuition but likely to appear through complex probabilistic mechanisms and systemic feedback:

• Non-linear social instability effects: in a context of demographic decline combined with increasing climate stress, unknown critical thresholds could generate sudden shifts in social cohesion, for example, local collapses in cascade, similar to critical phase transitions in statistical physics.
• Collective adaptive fatigue phenomena: the cumulative capacity of societies to adapt to repeated shocks (economic, climatic, health) could decrease exponentially, inducing increasing collective vulnerability unrelated to size or average income.
• Inverse demographic feedback effects: in some territories, the decline in the working-age population could paradoxically accelerate local environmental degradation (abandonment of cultivated land, collapse of infrastructure), causing a non-linear and self-reinforcing ecological and demographic vicious circle.
• Emerging risks related to global mental health: the simultaneous increase in loneliness in aging populations and climate instability could cause collective psychosocial stress phenomena, affecting productivity, innovation, and resilience, thus amplifying other systemic risks through a mechanism that is difficult to model but real.
• Unexpected technological disruptions: the growing dependence on automated systems to compensate for demographic decline could expose societies to cyber risks, systemic failures, or cascading failures, which could emerge probabilistically and destabilize critical infrastructures.

These so-called "unexpected" or "non-intuitive" risks often arise from the interaction of multiple coupled factors, generating high-dimensional dynamics where strange attractors or bifurcations may appear. Their probability of occurrence is low individually, but their potential impact is high enough to justify systemic integration into demographic and climate forecasting models.

A key step in evolution

The demographic transition marks a turning point in the history of humanity. It is not just a matter of numbers: it expresses the ability of the human species to emancipate itself from natural and environmental constraints, but also to collectively manage the effects of this emancipation. In this sense, it is fully part of the cultural evolution of humanity, alongside language, agriculture, and writing.