Are babies capable of engaging in deception? This study on “fake crying” in babies addresses a fascinating research question–the strategic use of deception by babies to get attention. From the abstract:
Two infants were observed longitudinally. In total, 102 crying episodes were analyzed. The infants displayed negative affect almost always just before starting to cry and soon after crying terminated. However, there were exceptions. Positive affect was observed. These were crying behaviors that the mother identified as “fake crying” or “emergence of fake crying”. These data indicate that, although normally infant affect just before and right after crying is negative, infants also can exhibit positive affect when they show fake crying. Infants who are capable of fake crying might communicate successfully with their caregivers.
So babies are smarter and more devious than they appear. We just wish the sample size were larger than n = 2. Also, as a related question, do babies ever engage in “fake smiling”?
Hat tips to digg.com, BPS Research Digest, and baby Adys Ann.
prior probability 
Babies definitely “fake cry” when they want something. I know my daughter will cry, even bawl if I put her down and then as soon as I pick her up nothing seems to be wrong with her anymore. Beautiful baby by the way! I don’t think my daughter “fake smiles.” I think she only genuinely smiles when she is happy.
When she is genuinely happy
Admittedly, the possibility of “fake smiling” in babies is hard to imagine … but not impossible (after all, the logic behind both forms of deception is the same — do x to get y), but how could we test this hypothesis?
Check out this study
http://moms.popsugar.com/Baby-Fake-Tears-33620603
http://thestir.cafemom.com/baby/167057/finally_proof_your_baby_is