Where does the Buddha say that human life can have at most 50% happiness? Ajahn Martin claims so in this talk: https://www.youtube.com/watch?v=Tsu3GEsoMT4 at 14:25. What's his source for this claim?
The Buddha say that human life can have at most 50% happiness?

That wasn't really what He meant. Basically He said that the reason why human realm is the most conducive to the cultivation of the Dhamma is because it's a realm where there's a mixed bag of misery and happiness, unlike realms where there's only exclusively just misery or just happiness.

&quot;It's a gain for you, monks, a great gain, that you've gained the opportunity to live the holy life. I have seen a hell named 'Six Spheres of Contact.' Whatever form one sees there with the eye is undesirable, never desirable; displeasing, never pleasing; disagreeable, never agreeable. Whatever sound one hears there with the ear... Whatever aroma one smells there with the nose... Whatever flavor one tastes there with the tongue... Whatever tactile sensation one touches there with the body... Whatever idea one cognizes there with the intellect is undesirable, never desirable; displeasing, never pleasing; disagreeable, never agreeable.
&quot;It's a gain for you, monks, a great gain, that you've gained the opportunity to live the holy life. I have seen a heaven named 'Six Spheres of Contact.' Whatever form one sees there with the eye is desirable, never undesirable; pleasing, never displeasing; agreeable, never disagreeable. Whatever sound one hears there with the ear... Whatever aroma one smells there with the nose... Whatever flavor one tastes there with the tongue... Whatever tactile sensation one touches there with the body... Whatever idea one cognizes there with the intellect is desirable, never undesirable; pleasing, never displeasing; agreeable, never disagreeable.
&quot;It's a gain for you, monks, a great gain, that you've gained the opportunity to live the holy life.&quot; ~~ SN 35.135 ~~

And Ven. Thanissaro's note:

The message here is that in realms where sense objects are totally disagreeable or totally agreeable it is very difficult to practice the holy life, for in the former, one is too distracted by pain; in the latter, too distracted by pleasure.