Americans should be the happiest people on earth. After all, the right to the “pursuit of happiness” is enshrined in our Declaration of Independence. “20/20” (9 p.m., ABC) talks to folks who think we’re all wrong about the ways we go about pursuing our inalienable right.

Correspondent Bob Weir looks at university courses on “Positive Psychology,” talks to experts claiming to study the “science” of happiness and discusses surveys ranking nations and societies by their sense of well-being. He also visits spots billed as “the happiest places on earth.” Let’s hope this isn’t another plug for Disneyland.

¢ As “Monk” (8 p.m., USA) enters the second half of its sixth season, Adrian Monk (Tony Shalhoub) goes the extra mile to find a young woman’s killer. After an eyewitness claims that she was ritually murdered by members of a local cult, Monk decides to infiltrate “The Siblings of the Sun” to investigate the charges and confront its charismatic leader (Howie Mandel, in a role he was born to play!).

A mass of neurosis, phobias and quirks, Monk is soon putty in the hands of the manipulative cult leader and his minions of blissed-out followers. Tortured by memories of his slain wife and continually stimulated by his keen powers of observation and deduction, he finds solace in a place where he can “just stop thinking.”

Funny, I thought that was why most people turned on their televisions.

¢ “Psych” (9 p.m., USA) enters the second half of its second season with a similarly silly whodunit. Shawn (James Roday) receives the shock of his life when Gus (Dule Hill) reveals that he was once briefly married and that his effervescent, eccentric and possibly unhinged former wife wants him back in her life.

At a time when police and detective dramas have become increasingly morbid (“CSI”), violent (“Law & Order: SVU”) , sadistic (“Criminal Minds”) and disturbing (“Dexter”), these two series keep the emphasis on clever, light comedy. OK, “Psych” treads a little too close to “Scrubs” in its quirk-quotient, but I like its emphasis on humor and friendship over mere shock value.

¢ Tom Arnold hosts the eight-part series “My Big Redneck Wedding” (8:30 p.m., CMT), accenting what the show is calling “rustic eccentricities” like receptions calling for mattress surfing and mud wrestling and canopies constructed out of beer cans.

Tonight’s other highlights

¢ Spirits matriculate at Ned’s new school on “Ghost Whisperer” (7 p.m., CBS).

¢ Bailey has a bad day on “Grey’s Anatomy” (7 p.m., ABC).

¢ The widow of comedian Richard Pryor needs help controlling foster dogs at her rescue shelter on “The Dog Whisperer” (7 p.m., National Geographic).

¢ Kidnappers snatch Josh on “Moonlight” (8 p.m., CBS).

¢ Brian feels pressure from recruiters on “Friday Night Lights” (8 p.m., NBC).

¢ A patient’s death sparks an investigation on “House” (8 p.m., Fox).

¢ Zoe detects competition on “Cashmere Mafia” (8 p.m., ABC).

¢ “Orangutan Island” (8 p.m., Animal Planet) enters the year in a new timeslot.

¢ Castoff pets face rehabilitation on “DogTown: Second Chances” (8 p.m., National Geographic).

¢ Charlie feels the heat on “Numb3rs” (9 p.m., CBS).

¢ A fugitive draws unwanted attention to the casino on “Las Vegas” (9 p.m., NBC).

¢ A damaged Hive ship may prove useful on “Stargate: Atlantis” (8 p.m., Sci Fi).