PSI linked

On occasion, in species with PSI abilities, twins or siblings will share the same psionic characteristics and thus are emotionally closely linked.

The chances of that occuring is roughly 1 in 10,000 for identical twins, 1 in 50,000 for semi-identical twins, 1 in 250,000 for non identical twins, and 1 in 500,000 for other siblings.

In rare cases, PSI Links can form between other relatives and in even rarer cases, they can form between two unrelated people.

Classified
There are a few known cases where non sentinent creatures from Green Hell have become PSI Linked to Greenies.

Roy Masters and Partner are also PSI Linked.

While the connection formed between Eric Olafson and The Narth at the time the former absorbed the later's Huhgavh in order for the later to survive, has been compared to at times to being similar to a PSI Link. Narth indicates it goes far beyond Psionics.