Introductory resources on developmental psychology/neuroscience I'd like to be able to answer questions such as:

What neuro/psychological functions underly competences such as empathy (or others - see background below)?
What neurotransmitters are of particular importance in those functions?
What genes are known to be associated with those neurotransmitters?

Any suggestions on textbooks or review papers that would be a good starting point?

Background
I'm an M.Sc. student in epidemiology (with a psychology undergrad) and my thesis is on &quot;gene x environment&quot; interactions (GxE). The outcome I'm interested in is child competence (construct which can be subdivided in subcomponents such as compliance, attention &amp; empathy) at 18 months. The main environmental predictor is mother sensitivity.
I now need to focus on some specific genes and would like to be able to choose those with a clear rationale (rather than, say, search through the literature to find the most commonly studied genes); However, at this point I lack the knowledge required to do so.
For an introduction to neurotransmitters (and the field of neuroscience) a good book to start with is Principles of Neural Science (Kandel, Schwartz, and Jessel, 2000). It is a standard reference used by many undergraduate and graduate courses in neuroscience/related fields. The next edition is due for publication in October. During development neurotransmitters act different than in adults, so you may also be interested on a review in this topic (Herlenius &amp; Lagercrantz, 2004); Although the time frame is earlier than what you are looking at.

In general I think you should start off by looking for possible neural mechanisms and any associated brain regions underlying the specific phenomenon you are interested in (in your example, empathy, or other subcomponents of social-emotional competence measured by the Infant-Toddler Social and Emotional Assessment (Carter et al., 2003)). I.e. Singer (2006) reviews the neural basis of empathy (as well as Theory of Mind); one section talks about implications for development.

After knowing what brain regions to focus on, you can look into anatomical/physiological research for better understanding of cellular type/structure, connectivity, synaptic characteristics, and neurotransmitters specific to these regions as well as genes that may affect normal function of a transmitter (i.e genes that control protein channels, enzymes/transmitter metabolism etc.). Here are two newer reviews, which link empathy to neuronal circuits and genes (Chakrabarti et al., 2009; Walter, 2012).