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World's most big-hearted nation: the United States

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The United States is the world's most generous nation, according to a global index of giving, as a higher proportion of Americans helped a stranger than any other country in the world.

The 2013 World Giving Index – published annually by international nonprofit organization Charities Aid Foundation – looks at three measures: monetary giving, volunteering and helping of strangers in a typical month. The survey, published on Tuesday, was carried out in 2012 across 135 countries.

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The U.S. made a significant improvement in charitable behavior, rising to the top position from fifth place last year.

Meanwhile, three countries tied for second place this year: Canada, Myanmar, and New Zealand. Australia, which was the highest-ranked nation in both the 2010 and 2012 reports, dropped to the seventh position.

Buffett & Gates on Charitable Giving & Chronic Disease Challenges
Buffett & Gates on Charitable Giving & Chronic Disease Challenges

Rounding out the bottom of the World Giving Index was Greece, reflecting the sluggish economic recovery in the indebted euro zone nation.

Overall, the index found that the world had become a more generous place, with the percentage of people donating money, volunteering time, and helping a stranger growing in 2012 compared with 2011.

The average proportion of people helping strangers across the countries surveyed in 2012 was 47 percent, up 2.4 percentage points from 2011.

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The next highest increase was in volunteering time, up 1.4 percentage points, at 19.7 percent.

And finally, the number of people donating money was 28.5 percent, an increase of 1.3 percentage points.

While levels of giving remain below those recorded five years ago, emerging economies are surging ahead in charitable donations. In India, for example, 244 million people give money to charity in a typical month, up from 163 million in the previous year.

According to the survey findings, there are gender differences when it comes to giving patterns.

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Women are more likely to donate money to charity than men – a global trend that has been in place since 2008. This comes despite a reported worsening of the imbalance between men and women in terms of global employment.

On average, 28.7 percent of women globally give to charity in a typical month, 0.9 percentage points ahead of the equivalent figure for men.

—By CNBC's Ansuya Harjani; Follow her on Twitter @Ansuya_H