Mui tsai was a product of prevailing Chinese cultural prejudices towards the birth of a girl in a family, which in their most extreme form expressed themselves in female infanticide.
Mui tsai, which means 'little sister' in Cantonese, was the South China version of a practice in which, if they were more fortunate, girls were purchased by rich families from poor families to work in their households as servants, with board and lodging and clothing provided. They would then be married off by this same family when they reached adulthood, usually around the age of eighteen, and many became daughters-in-law.
Less fortunate girls were abused, mistreated and resold, or trained by women of ill repute under the direction of pimps and middlemen to work as prostitutes in flower boats, houses of pleasure and tea gardens.
The mui tsai system was variously regarded, from being just a matter of working conditions, to being a system of trafficking and recruitment into prostitution and to child slavery.
When the British occupied Hong Kong in 1841 they proclaimed that the Chinese would be governed according to their own laws and customs, but this aspiration changed as attitudes changed in the West. Registration of births and deaths came in 1872 and marked a beginning of addressing the problem.
After the 1911 revolution in China, the mui tsai system was banned there, but this was never effectively enforced.
In the 1920s and 1930s, there was an increased interest in social welfare in the West. In 1922, the Secretary for the Colonies, Winston Churchill argued to the House of Commons that the system should be abolished.
In 1923 the Female Domestic Service Ordinance was passed in Hong Kong in an attempt to ban the practice and in 1929 there was an amendment to bring in registration of existing mui tsai. Over 4000 girls were registered but it was claimed this was but half the actual total.
The 1926 Slavery Convention was a treaty established under the League of Nations, banning slave trade and slavery, to which Britain as a signatory of the Convention brought international attention to anti-mui tsai efforts. But old customs die hard, especially in rural areas.
In the 1950s and 60s attitudes were changing in Hong Kong but influxes of Chinese from the mainland brought old attitudes back.
With the passing of the International Covenant on Civil and Political Rights in 1976, Hong Kong began forming Human Rights laws around human trafficking.
Today, although the mui tsai question has been dealt with through legislation, some of the issues are still current. There are cases of cruelty to domestic helpers, punished by employers who still believe their maids belong to them. There are elderly women in Hong Kong who were sold into servitude as infants.
Although the mui tsai system is no longer accepted, many of the factors underlying its development such as poverty and son preference or discriminations against the female gender are still prevalent.
Sources:
The Yip Family of Amah Rock by Jill Doggett
Wikipedia