A woman's kindness - or nastiness - is largely due to the genes she inherited from her mum and dad.
But for men it comes from the environment. That's the conclusion of a new scientific study addressing the age-old nature versus nurture debate.
New Zealand researchers found women were more naturally kind, while men's compassion was more dependent on upbringing, family and friends.
"It means that women have a much stronger internal moral compass than men do. For women that compass seems to be more built-in and more independent of the environment," said Kiwi psychologist Professor Tim Bates.
To reach the result researchers from Edinburgh University compared almost 1000 sets of adult identical and non-identical twins.
This meant they could tease apart the role of genes, as the identical twins shared both family and genes, while non-identical twins shared family and only half of their genes.
The twins all answered 11 questions about the level of obligation they felt in a range of work, civic and welfare situations, such as serving on a jury, voting in elections, and collecting money for charity.
The researchers found same-sex twins, especially females, were more similar in their results than the non-identical twins, showing the importance of genes.
Their results showed half of women's kindness, 48 per cent, was due to their genes, compared to only a fifth in men.
Women were also kinder overall, scoring higher than men in all three areas.
Acts of kindness were central to the function of communities, said Bates.
Siblings' scientific experiment
Bianca Richardson-Read always thought she was nicer than her Shortland Street star brother - and now, she reckons, science confirms it.
Bianca, 24, and Tyler, 19, who plays Evan Cooper on the TV show, answered the same 11 questions used by psychologists in their nature vs nurture project.
Bianca scored 76 out of a possible 110 for kindness - a tad above her brother's 74.
"I would agree with that - I'm definitely the nicest in the family," she laughed.
For his part, Tyler said he was surprised to see the results. "But since it's quite close I guess we're quite similar," he said.
The siblings agreed with the study's findings that genes played a greater role in kindness for women than men.
"The family that I've grown up with has really shaped me," said Tyler.