longevity and declining fertility. An increase in longevity raises the average age of the population by increasing the numbers of surviving older people. A decline in fertility reduces the number of babies, and as the effect continues, the numbers of younger people in general also reduce. Of the two forces, declining fertility now contributes to most of the population ageing in the world. More specifically, the large decline in the overall fertility rate over the last half-century is primarily responsible for the population ageing in the world's most developed countries. Because many developing countries are going through faster fertility transitions, they will experience even faster population ageing than the currently-developed countries will. The rate at which the population ages is likely to increase over the next three decades; however, few countries know whether their older population are living the extra years of life in good or poor health. A "compression of morbidity" would imply reduced disability in old age, but an expansion would see an increase in poor health with increased longevity. Another option has been posed for a situation of "dynamic equilibrium." That is crucial information for governments if the limits of lifespan continue to increase indefinitely, as some researchers believe. The World Health Organization's suite of household health studies is working to provide the needed health and well-being evidence, such as the World Health Survey, and the Study on Global Ageing and Adult Health (SAGE). The surveys cover 308,000 respondents aged at least 18 and 81,000 aged at least 50 from 70 countries. The Global Ageing Survey, exploring attitudes, expectations, and behaviours towards later life and retirement, directed by George Leeson, and covering 44,000 people aged 40–80 in 24 countries from across the globe, has revealed that many people are now fully aware of the ageing of the world's population and the implications that it will have on their lives and those of their children and grandchildren. Canada has the highest per capita immigration rate in the world, partly to counter population ageing. The C. D. Howe Institute, a conservative think tank, has suggested that immigration cannot be used as a viable means to counter population ageing. That conclusion is also seen in the work of other scholars. The demographers Peter McDonald and Rebecca Kippen commented, "As fertility sinks further below replacement level, increasingly higher levels of annual net migration will be required to maintain a target of