The result was The Happy Song. Caspar Addyman, an expert in child development, and Lauren Stewart, a musical psychologist, were asked by U.K. baby food manufacturer Cow & Gate to analyze existing science about infant preferences in order come up with the ideal parameters for a baby-friendly pop hit—one that was “proven to make babies happy.” 
 
 After extensive research, the two hired Grammy-winning composer and vocalist Imogen Heap — formerly of the synth-pop duo Frou Frou and mother to an 18-month-old girl — to put together a melody following some ground rules: it had to be in a major key, be simple and repetitive, and have a broad range of dynamics to keep the baby surprised and tuned in. The song also had to be uptempo, mirroring a baby’s heartbeat, which is faster than a human’s, and it had to have a lively female vocal track, “ideally recorded in the presence of an actual baby.” Previous studies have shown that babies tend to be partial to the sing-song tones that women often use around infants. But we also filmed the babies’ responses and coded the videos for laughs, smiles and dancing. “The secret was to make it silly and make it social,” Addyman say, so they surveyed Cow & Gate customers to find out which sounds made their babies laugh. We also know babies respond better to “plosive” vocal sounds like “pa” and “ba” compared to “sonorant” sounds like “la”.
– Have a fussy baby? Researchers believe they have the solution: a simple lullaby specifically designed to make babies happy. Though it's a simple tune, the process behind "The Happy Song" was actually quite complicated, per Time. The song is the result of plenty of research into infant musical tastes, which revealed infants would likely respond best to an upbeat song in a major key that was simple and repetitive, but also used various musical devices for an element of surprise, child development expert Caspar Addyman writes at the Conversation. Once a list of requirements was agreed upon, Addyman and musical psychologist Lauren Stewart enlisted Grammy winner Imogen Heap, who has an 18-month-old daughter, to create four test melodies. Two were slow and two were fast. When played for parents and their babies, the majority favored one upbeat tune in particular. Heap then included favorite sounds voted on by parents, including "boo," animal noises, and laughter, along with her own high-energy voice and plosive sounds with pop, like "pa" and "ba," which babies seem to prefer, according to a YouTube video. When 20 infants heard the final version of "The Happy Song," they were "entranced," Addyman writes. This "wasn't the most scientific as tests go," but "I can definitely, confidently say from a scientific point of view something's happened here." Researchers next plan to look at physiological responses to the song, which came in at 163 beats per minute. Listen to "The Happy Song" here. (This is the world's oldest known melody.)