Do we need to review our long-standing policy of one-way permits for mainlanders settling in Hong Kong? Yes, absolutely. Does its quota of 150 daily permits have an impact on our housing supply? Probably. But is it the root cause of our chronic housing shortage? Absolutely not; and it is disingenuous for pan-democrat lawmakers Gary Fan Kwok-wai and Claudia Mo Man-ching to suggest it is. In a controversial advertisement the pair ran last month in several local newspapers and one in Taiwan, they called for changes to the decades-long policy. In defending their ad yesterday, Fan said the policy was at "the root of the housing problem". Any fair-minded person knows housing is a complicated and long-standing problem with myriad causes. Chief among these has been the government's high land premium policy to keep revenue up and taxes down. It can do it because it owns all the land in Hong Kong. The ultra-low-interest environment - sometimes reaching negative real interest rates in the past few years - helped fuel the property bubble and priced many people out of the market. The Leung Chung-ying administration aims to provide 470,000 flats over the next decade, an ambitious target unlikely to be met. This is because we have voluntarily handicapped ourselves. Officials are unwilling to take on indigenous villagers to reverse the small house policy. The public doesn't want any development in country parks. Everyone is afraid to ask the People's Liberation Army and the central government to cough up land from underused barracks. Sea reclamation is controversial and the rezoning of industrial, government and public land is difficult. Given our trouble, our default position is to blame mainlanders for trying to take up space in Hong Kong. But even if we scrap all permit quotas, it's unclear what impact that might have on housing supply. But it would be against every humanitarian principle as most permits go to mainlanders seeking family reunion in Hong Kong. It's hard to avoid the impression that Fan and Mo are just exploiting populist resentments against mainlanders.