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Developed countries have experienced both some population growth and unprecedented declines in fertility rates during the last half of the twentieth century. Couples now have fewer than two children on average in most European countries and they tend to postpone these births until a later age. A decline in male fertility has been suggested by some studies of semen quality, but there is contrasting evidence of shorter times to pregnancy for couples trying to conceive. An important economic factor is the income of young men relative to their parents' incomes, which determines how they rate the ability of their own earnings to support a family. Lower relative income in the 1970s was associated with a lower fertility rate. The decline in fertility in the USA may have been attenuated by the sharp rise in female income during the late 1960s and early 1970s, allowing women to take advantage of purchased child care, thus maintaining the relative family income. The level of demand for children does not appear to be set by known psychological factors, although explanations for the desire to reproduce have been sought in biological, psychoanalytical and socio-cultural research. Recent studies indicate that adults with secure attachment relationships are more interested in being parents. Possible epidemiological factors include age at first marriage, but in Eastern Europe, where age at first marriage is as low as 22 years, fecundity rates do not exceed 1.5. When mothers' age cohorts are analysed, the mean fecundity rate has been falling since the 1920s. Health factors affecting population trends include the change in contraceptive prevalence over the last 40 years. The prevalence of sub-fertility remains close to 10%, and studies from a number of countries indicate that approximately 50% of infertile couples make use of infertility services including IVF and intracytoplasmic sperm injection which are available in 45 countries covering 78% of the world's population. It is estimated that the level of service is sufficient for less than one-third of the need.