The 1971 census recorded an increase in population compared to the pervious censuses and to counter the issue of growing population family planning methods were introduced.
The 42nd Amendment picked the 1971 Census data as base for all calculations and froze it till 2001 Census.
This is to equalise the shares and safeguard the states actively undertook the family planning programme.
In 2001, the 84th Amendment further extended that to the first Census after 2026, which will be the Census of 2031.
What are the concerns of the states on this move?
Northern states like Uttar Pradesh, Maharashtra and Bihar have more than doubled their population in the intervening years.
Whereas southern states like Tamil Nadu, Karnataka and Kerala have had relatively slower growthsin population with an exception of Andhra Pradesh.
The usage of the 2011 Census is being opposed for the same reason the usage of 1971 Census was made mandatory.
Which made sure states that have worked on population control do not lose out on benefits.
Thus according to the southern states the decision of the 15th FC to use the 2011 census data is seen as discriminatory and affects the federal structure.