At the annual meeting of the American Association for the Advancement of Science she said: "The availability of energy has been a major factor in population growth.

"In the modern context, energy use per capita affects economic activity. So a prolonged decline in energy use per capita will tend to depress the economy which, in turn, will cause a decline in the fertility rate."

Despite continuing high consumption, experts had calculated that production of petroleum and liquid natural gas will reach a plateau and then decline within five to 10 years.

Such a downturn would have a major economic impact, said Professor Abernethy, from Vanderbilt University in Nashville, Tennessee.

People tended to increase the size of their families when they believed economic opportunities were expanding, she told the conference in Seattle.

Conversely, families shrank in size when people thought their resources were diminishing, making it more difficult to raise children.

The energy-fertility link theory was backed by historical evidence, said Professor Abernethy.

The 15-year "baby boom" that followed the Second World War was accompanied by a substantial increase in technology-driven productivity.

An increasingly large and affluent middle class emerged that responded with "early marriage and closely spaced births".

The baby boom ended within a year following rising energy prices and the 1961 recession.

Professor Abernethy said: "The improving standard of living to which many societies have become accustomed will be difficult to maintain in the face of rapidly rising prices for energy.

"In these circumstances, fertility rates are unlikely to rise. Indeed, a future marked by declining energy use per capita may be the ultimate driver of worldwide declines in fertility."

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