Hindu reincarnationists claim that some souls have transmigrated from insects and animals, thus giving us more human souls. However, New Age reincarnationists do not believe in the transmigration of the soul from insect to human—only human to human. So, this isn’t a solution for them.
Population growth renders a problem for the reincarnationist: If souls are eternal and no new souls are being created, then where are these extra souls coming from? Remember, some souls at least are being absorbed into Brahman. Therefore, the human population should be decreasing not increasing.