The court heard arguments on challenges to laws in West Virginia and Idaho that bar transgender students from participating in girls' or women's sports.
Roberts didn't directly address frequent criticism of judges who have ruled against the Trump administration or the continuing increase in threats against them.
More than 100 pizzas were delivered to the homes of judges and their families this year, some with signs of foreign involvement. Judges say the message is clear: We know where you live.
The Trump administration is seeking to roll back the historical understanding that birthright citizenship extends to almost anyone born in the United States.