Cities act like giant contraceptives and could help curb the Earth's explosive population growth that drives climate change, according to two studies.

The United Nations estimates that in the past year the majority of the world's population will, for the first time in human history, live in town and cities.

Cities are now home to half of the world's 6.6 billion humans and by 2030 that urban fraction will rise to 60 per cent as nearly 5 billion people will live in cities of the projected global population of 8.2 billion.

From the times of the Romans, urban parents' biggest worry about their children used to be that their babies would not survive infancy, as a result of lack of sanitation and disease, but this has given way to a concern that the children won't be successful as adults, says Prof Ruth Mace of University College London.

Rather than following the usual evolutionary line that individuals with more resources have more offspring, something different now happens as a result: parents are putting more resources into each child so they can compete with their peers, and this leads parents to have fewer children, she says in the journal Science.

She likens the effect to one seen in the animal kingdom, where the competition for mates can generate "runaway selection," as in the case of the male peacock's extravagant tail.

Similarly, the conditions of modern urban life seem to have generated a runaway process of ever-escalating levels of investment in our children, Prof Mace says, which could continue to drive birth rates ever lower.

Italy and Mexico are examples of countries where this effect is clearly damping down birth rate but she says the fertility decline in Africa is also happening fastest in urban areas.

"In Ethiopia there is a difference of nearly four children between family sizes in the capital city (Addis Ababa) and in rural areas that is the highest such difference seen anywhere," says Prof Mace.

Meanwhile another study in the journal Science from Iceland suggests that the worldwide trend towards urbanisation could slow population growth because it offers a wider choice of potential partners. The reason is that married cousins have more children than do parents with closer or more distant relationships.

In Iceland, marriages between third and fourth cousins tend to produce more children and grandchildren than other marriages, and this trend seems to have a biological basis, according to the study by the Icelandic pharmaceutical company deCODE genetics.

Previous studies have suggested that related couples tend to have more children than unrelated couples, but they weren't able to rule out the possibility that socioeconomic factors - such as the advantages of preserving land and wealth within extended families - were responsible for the difference.

Now Dr Agnar Helgason and colleagues at deCODE in Reykjavik have found that this is true by studying population-wide genealogical data in Iceland, home to one of the most socioeconomically and culturally homogenous societies in the world.

They mapped out the degrees of kinship among all known Icelandic couples whose members were born between 1800 and 1965, and they compared the numbers of children and grandchildren descended from these couples.

The results showed that couples related at the level of third and fourth cousins had the most children, which may reflect a balance between the disadvantages of inbreeding and the benefits of keeping some degree of genetic compatibility between mother and developing child, though they admit that the fundamental reason is not yet known.

The work suggests that the recent and dramatic demographic shift experienced in Iceland - from a rural society to a highly urbanised one - may serve to slow population growth, as individuals are exposed to a much broader range of distantly related potential mates.

If so, this could be of relevance to slowing population growth in the many countries around the world making the transition from closely-knit rural societies, where cousins are more likely to marry, to more urbanised ones.

The study shows for example that for women born between 1800 and 1824, those with a partner related at the level of a third cousin had an average of 4.04 children and 9.17 grandchildren, while those related to their mates as eighth cousins or more distantly had 3.34 children and 7.31 grandchildren.

For women born in the period 1925-1949 with mates related at the degree of third cousins, the average number of children and grandchildren were 3.27 and 6.64, compared to 2.45 and 4.86 for those with partners who were eighth cousins or more distantly related.

The findings hold for those born in the year 1800 up to the present day, spanning eras when Iceland was a predominantly poor and rural country, to the present-day era of a highly urbanised society with one of the highest standards of living in the world.