Through the journal entries of Theo in The Children of Men we are provided with an in-universe perspective of the change in humanities spirit in P.D. James grim vision of the future. 

Through Theo’s eyes passion has died in the face of modernity and disillusionment.  Presaging the loss of man’s ability to reproduce he witnessed the slow death of the great ocean that is humanities ability to feel.  But feel what?  The sensations of the flesh?   No, but Theo saw the loss of desire even before the Omega generation was conceived.  The drives of the body are co-morbid with the state of the mind.  So the root of the moratorium of humanities spirit must be elsewhere. 
The answer I believe is in the loss of not just the ability to love, but the ability to feel with passion.  Humanity did not lose just one hue of the spectrum of passions.  Humanity lost all of them.  There is no great war.  There is no longer place for the youth, the Omega’s, to prove their strength.  Passionate love and hate, fear and courage, these things are gone.   In their place what is left?  Nothing.  Moratorium of the spirit overtook humanity even before Omega in Theo's eyes.