IB Diploma Programme Geography SL (first assessment 2019; 2017 guide). The core is shared by SL and HL, and this topic covers its first unit in full — Population distribution—changing population — from the measures of population change (crude birth and death rates, natural increase) through the demographic transition model and population structure to migration, ageing and youthful populations, and population policy. The geographic concepts (POWER, PROCESS, POSSIBILITY, SCALE, SPATIAL INTERACTIONS) frame every answer, and the worked numbers below are original and reconciled.

Must-know for the changing-population unit:

Population distribution answers WHERE people live and WHY: the physical frame (climate, water, relief, soils), the economic frame (jobs, trade routes, cities), and history and politics (colonisation, conflict, policy). Distribution is described by DENSITY — arithmetic density $=$ population ÷ area; physiological density $=$ population ÷ arable area, exposing pressure on farmland. Sparse and dense are relative, and the WHY questions carry the marks: altitude and aridity thin populations; industry and coastlines thicken them.

The MEASURES of change: the crude birth rate 粗出生率 (CBR) is live births per 1 000 people per year; the crude death rate 粗死亡率 (CDR) is deaths per 1 000 per year; natural increase 自然增长 is their difference — a country with CBR 18 and CDR 9 has NIR $= 18 - 9 = 9$ per 1 000 $= 0.9\%$ a year, and a doubling time of about $70 / 0.9 \approx 78$ years (the rule of 70). TOTAL population change adds NET MIGRATION to natural increase: growth is births minus deaths plus incomers minus outgoers.

The demographic transition model 人口转变模型 (DTM) traces countries through five stages. Stage 1: high CBR and CDR, low growth (no country remains). Stage 2: death rates fall (medicine, sanitation) while births stay high — explosive growth, the classic LEDC profile. Stage 3: birth rates fall (education, urbanisation, women's work) — growth slows. Stage 4: both low, growth near zero — most MEDCs. Stage 5: births below replacement (CBR below CDR) — decline, as in Japan and Germany. The model's limits: it generalises from Europe's path, undervalues migration, and dates — but as a frame for comparing countries it remains the exam's workhorse.

POPULATION STRUCTURE is read from the population pyramid 人口金字塔: an EXPANSIVE pyramid (wide base) means high fertility and a youthful population (Nigeria-shape); a STATIONARY pyramid means near-replacement growth; a CONSTRICTIVE pyramid (narrow base, wide top) means ageing and decline (Japan-shape). The dependency ratio 抚养比 summarises the load: dependents (0–14 plus 65+) over the working group (15–64), times 100 — with 60 children, 30 elderly and 210 workers the ratio is $90/210 \times 100 \approx 43$ per 100. AGEING populations strain pensions and health care while labour shortens; YOUTHFUL populations strain schooling and jobs while promising a future demographic dividend; the two cases are evaluated, not just described.

POPULATION POLICY answers both: pro-natalist policy 鼓励生育政策 (France's childcare, leave and cash support) raises fertility in ageing societies; anti-natalist policy (China's one-child era, 1979 to 2015) lowered fertility at the price of an ageing population and a skewed sex ratio; some countries now pursue population-neutral policies on health, education and women's employment, letting fertility follow.

Migration 迁移 is a move crossing a boundary for more than a year: INTERNAL (rural to urban) or INTERNATIONAL, voluntary or forced. The explanations are push factors 推力因素 driving people out (unemployment, conflict, disaster, land shortage) and PULL factors drawing them in (jobs, safety, family, services), moderated by BARRIERS (cost, distance, borders, language) — Lee's model adds intervening obstacles and personal factors to the two lists. FORCED migrants who cross a border are refugees 难民 (1951 Convention: well-founded fear of persecution); those displaced within their state are IDPs; economic migrants move by choice and fall outside the Convention. IMPACTS flow both ways: origin communities lose the young and gain remittances (money sent home, a major GDP line for Nepal and the Philippines), relieve unemployment and receive return migration's skills; destinations gain labour and youth, and absorb integration costs and social strain. Internal migration drives urbanisation and, at scale, MEGACITY growth — the process the urban-environments option takes up.

Worked example. A district of 2 million with CBR 24 and CDR 8 grows by natural increase of 16 per 1 000 a year; net out-migration of 12 per 1 000 (young adults leaving for the coast) leaves total growth near 4 per 1 000 — a population ageing even as it grows slowly, the exact profile that switches policy debate from jobs to elder care.

• Distribution factors physical, economic, historical, political; arithmetic vs physiological density (distribution).
• CBR/CDR per 1 000; NIR $=$ CBR $-$ CDR (18 − 9 → 0.9%); doubling time $\approx 70/$NIR (≈78 years); add net migration for total change (measures).
• DTM stages 1–5 with the CBR/CDR logic of each, and two limitations of the model (transition).
• Pyramid shapes (expansive, stationary, constrictive) with country profiles; dependency ratio 90/210 ≈ 43; ageing vs youthful strain evaluated both ways (structure).
• Pro-natalist (France), anti-natalist (China 1979–2015) and neutral policies with their trade-offs (policy).
• Push/pull/barriers (Lee); refugees vs IDPs vs economic migrants (1951 Convention); remittances and the two-sided impacts; internal migration → urbanisation (migration).

Boundary notes: the SL core paper rewards command-term precision — "explain" needs a because-chain, "examine" needs strengths and limits; data-response items calculate NIR, doubling time and dependency ratios from given figures. All worked numbers are original and reconciled.